diff --git a/src/create/document.test.ts b/src/create/document.test.ts
index 76ed050..fd435f9 100644
--- a/src/create/document.test.ts
+++ b/src/create/document.test.ts
@@ -646,304 +646,304 @@ describe('createDocument', () => {
     });
 
     expect(document).toMatchInlineSnapshot(`
-      {
-        "components": {
-          "headers": {
-            "my-header": {
-              "required": true,
-              "schema": {
-                "type": "string",
-              },
-            },
+{
+  "components": {
+    "headers": {
+      "my-header": {
+        "required": true,
+        "schema": {
+          "type": "string",
+        },
+      },
+    },
+    "parameters": {
+      "b": {
+        "in": "path",
+        "name": "b",
+        "required": true,
+        "schema": {
+          "type": "string",
+        },
+      },
+    },
+    "schemas": {
+      "a": {
+        "type": "string",
+      },
+      "b": {
+        "properties": {
+          "a": {
+            "type": "string",
           },
-          "parameters": {
-            "b": {
-              "in": "path",
-              "name": "b",
-              "required": true,
-              "schema": {
-                "type": "string",
-              },
-            },
+        },
+        "required": [
+          "a",
+        ],
+        "type": "object",
+      },
+      "c": {
+        "allOf": [
+          {
+            "$ref": "#/components/schemas/b",
           },
-          "schemas": {
-            "a": {
-              "type": "string",
-            },
-            "b": {
-              "properties": {
-                "a": {
-                  "type": "string",
-                },
-              },
-              "required": [
-                "a",
-              ],
-              "type": "object",
-            },
-            "c": {
-              "allOf": [
-                {
-                  "$ref": "#/components/schemas/b",
-                },
-              ],
-              "properties": {
-                "d": {
-                  "nullable": true,
-                  "type": "string",
-                },
-              },
-              "required": [
-                "d",
-              ],
-              "type": "object",
-            },
-            "lazy": {
-              "items": {
-                "$ref": "#/components/schemas/lazy",
-              },
-              "type": "array",
-            },
-            "manual": {
-              "type": "boolean",
-            },
-            "post": {
-              "properties": {
-                "id": {
-                  "type": "string",
-                },
-                "user": {
-                  "$ref": "#/components/schemas/user",
-                },
-                "userId": {
-                  "type": "string",
-                },
-              },
-              "required": [
-                "id",
-                "userId",
-              ],
-              "type": "object",
-            },
-            "union-a": {
-              "properties": {
-                "type": {
-                  "enum": [
-                    "a",
-                  ],
-                  "type": "string",
-                },
-              },
-              "required": [
-                "type",
-              ],
-              "type": "object",
-            },
-            "union-b": {
-              "properties": {
-                "type": {
-                  "enum": [
-                    "b",
-                  ],
-                  "type": "string",
-                },
-              },
-              "required": [
-                "type",
-              ],
-              "type": "object",
+        ],
+        "properties": {
+          "d": {
+            "nullable": true,
+            "type": "string",
+          },
+        },
+        "required": [
+          "d",
+        ],
+        "type": "object",
+      },
+      "lazy": {
+        "items": {
+          "$ref": "#/components/schemas/lazy",
+        },
+        "type": "array",
+      },
+      "manual": {
+        "type": "boolean",
+      },
+      "post": {
+        "properties": {
+          "id": {
+            "type": "string",
+          },
+          "user": {
+            "$ref": "#/components/schemas/user",
+          },
+          "userId": {
+            "type": "string",
+          },
+        },
+        "required": [
+          "id",
+          "userId",
+        ],
+        "type": "object",
+      },
+      "union-a": {
+        "properties": {
+          "type": {
+            "enum": [
+              "a",
+            ],
+            "type": "string",
+          },
+        },
+        "required": [
+          "type",
+        ],
+        "type": "object",
+      },
+      "union-b": {
+        "properties": {
+          "type": {
+            "enum": [
+              "b",
+            ],
+            "type": "string",
+          },
+        },
+        "required": [
+          "type",
+        ],
+        "type": "object",
+      },
+      "user": {
+        "properties": {
+          "id": {
+            "type": "string",
+          },
+          "posts": {
+            "items": {
+              "$ref": "#/components/schemas/post",
             },
-            "user": {
-              "properties": {
-                "id": {
-                  "type": "string",
-                },
-                "posts": {
-                  "items": {
-                    "$ref": "#/components/schemas/post",
+            "type": "array",
+          },
+        },
+        "required": [
+          "id",
+        ],
+        "type": "object",
+      },
+    },
+  },
+  "info": {
+    "title": "My API",
+    "version": "1.0.0",
+  },
+  "openapi": "3.0.0",
+  "paths": {
+    "/jobs": {
+      "get": {
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/b",
+          },
+        ],
+        "requestBody": {
+          "content": {
+            "application/json": {
+              "schema": {
+                "properties": {
+                  "a": {
+                    "$ref": "#/components/schemas/a",
+                  },
+                  "b": {
+                    "$ref": "#/components/schemas/b",
+                  },
+                  "c": {
+                    "$ref": "#/components/schemas/b",
+                  },
+                  "d": {
+                    "$ref": "#/components/schemas/c",
+                  },
+                  "e": {
+                    "discriminator": {
+                      "mapping": {
+                        "a": "#/components/schemas/union-a",
+                        "b": "#/components/schemas/union-b",
+                      },
+                      "propertyName": "type",
+                    },
+                    "oneOf": [
+                      {
+                        "$ref": "#/components/schemas/union-a",
+                      },
+                      {
+                        "$ref": "#/components/schemas/union-b",
+                      },
+                    ],
+                  },
+                  "f": {
+                    "items": {
+                      "oneOf": [
+                        {
+                          "type": "string",
+                        },
+                        {
+                          "type": "number",
+                        },
+                        {
+                          "$ref": "#/components/schemas/manual",
+                        },
+                      ],
+                    },
+                    "maxItems": 3,
+                    "minItems": 3,
+                    "type": "array",
+                  },
+                  "g": {
+                    "$ref": "#/components/schemas/lazy",
+                  },
+                  "h": {
+                    "$ref": "#/components/schemas/user",
                   },
-                  "type": "array",
                 },
+                "required": [
+                  "a",
+                  "b",
+                  "d",
+                  "e",
+                  "f",
+                  "g",
+                  "h",
+                ],
+                "type": "object",
               },
-              "required": [
-                "id",
-              ],
-              "type": "object",
             },
           },
         },
-        "info": {
-          "title": "My API",
-          "version": "1.0.0",
-        },
-        "openapi": "3.0.0",
-        "paths": {
-          "/jobs": {
-            "get": {
-              "parameters": [
-                {
-                  "$ref": "#/components/parameters/b",
-                },
-              ],
-              "requestBody": {
-                "content": {
-                  "application/json": {
-                    "schema": {
-                      "properties": {
-                        "a": {
-                          "$ref": "#/components/schemas/a",
-                        },
-                        "b": {
-                          "$ref": "#/components/schemas/b",
-                        },
-                        "c": {
-                          "$ref": "#/components/schemas/b",
-                        },
-                        "d": {
-                          "$ref": "#/components/schemas/c",
-                        },
-                        "e": {
-                          "discriminator": {
-                            "mapping": {
-                              "a": "#/components/schemas/union-a",
-                              "b": "#/components/schemas/union-b",
-                            },
-                            "propertyName": "type",
-                          },
-                          "oneOf": [
-                            {
-                              "$ref": "#/components/schemas/union-a",
-                            },
-                            {
-                              "$ref": "#/components/schemas/union-b",
-                            },
-                          ],
-                        },
-                        "f": {
-                          "items": {
-                            "oneOf": [
-                              {
-                                "type": "string",
-                              },
-                              {
-                                "type": "number",
-                              },
-                              {
-                                "$ref": "#/components/schemas/manual",
-                              },
-                            ],
-                          },
-                          "maxItems": 3,
-                          "minItems": 3,
-                          "type": "array",
+        "responses": {
+          "200": {
+            "content": {
+              "application/json": {
+                "schema": {
+                  "properties": {
+                    "a": {
+                      "$ref": "#/components/schemas/a",
+                    },
+                    "b": {
+                      "$ref": "#/components/schemas/b",
+                    },
+                    "c": {
+                      "$ref": "#/components/schemas/b",
+                    },
+                    "d": {
+                      "$ref": "#/components/schemas/c",
+                    },
+                    "e": {
+                      "discriminator": {
+                        "mapping": {
+                          "a": "#/components/schemas/union-a",
+                          "b": "#/components/schemas/union-b",
                         },
-                        "g": {
-                          "$ref": "#/components/schemas/lazy",
+                        "propertyName": "type",
+                      },
+                      "oneOf": [
+                        {
+                          "$ref": "#/components/schemas/union-a",
                         },
-                        "h": {
-                          "$ref": "#/components/schemas/user",
+                        {
+                          "$ref": "#/components/schemas/union-b",
                         },
-                      },
-                      "required": [
-                        "a",
-                        "b",
-                        "d",
-                        "e",
-                        "f",
-                        "g",
-                        "h",
                       ],
-                      "type": "object",
                     },
-                  },
-                },
-              },
-              "responses": {
-                "200": {
-                  "content": {
-                    "application/json": {
-                      "schema": {
-                        "properties": {
-                          "a": {
-                            "$ref": "#/components/schemas/a",
-                          },
-                          "b": {
-                            "$ref": "#/components/schemas/b",
-                          },
-                          "c": {
-                            "$ref": "#/components/schemas/b",
-                          },
-                          "d": {
-                            "$ref": "#/components/schemas/c",
-                          },
-                          "e": {
-                            "discriminator": {
-                              "mapping": {
-                                "a": "#/components/schemas/union-a",
-                                "b": "#/components/schemas/union-b",
-                              },
-                              "propertyName": "type",
-                            },
-                            "oneOf": [
-                              {
-                                "$ref": "#/components/schemas/union-a",
-                              },
-                              {
-                                "$ref": "#/components/schemas/union-b",
-                              },
-                            ],
-                          },
-                          "f": {
-                            "items": {
-                              "oneOf": [
-                                {
-                                  "type": "string",
-                                },
-                                {
-                                  "type": "number",
-                                },
-                                {
-                                  "$ref": "#/components/schemas/manual",
-                                },
-                              ],
-                            },
-                            "maxItems": 3,
-                            "minItems": 3,
-                            "type": "array",
+                    "f": {
+                      "items": {
+                        "oneOf": [
+                          {
+                            "type": "string",
                           },
-                          "g": {
-                            "$ref": "#/components/schemas/lazy",
+                          {
+                            "type": "number",
                           },
-                          "h": {
-                            "$ref": "#/components/schemas/user",
+                          {
+                            "$ref": "#/components/schemas/manual",
                           },
-                        },
-                        "required": [
-                          "a",
-                          "b",
-                          "d",
-                          "e",
-                          "f",
-                          "g",
-                          "h",
                         ],
-                        "type": "object",
                       },
+                      "maxItems": 3,
+                      "minItems": 3,
+                      "type": "array",
                     },
-                  },
-                  "description": "200 OK",
-                  "headers": {
-                    "my-header": {
-                      "$ref": "#/components/headers/my-header",
+                    "g": {
+                      "$ref": "#/components/schemas/lazy",
+                    },
+                    "h": {
+                      "$ref": "#/components/schemas/user",
                     },
                   },
+                  "required": [
+                    "a",
+                    "b",
+                    "d",
+                    "e",
+                    "f",
+                    "g",
+                    "h",
+                  ],
+                  "type": "object",
                 },
               },
             },
+            "description": "200 OK",
+            "headers": {
+              "my-header": {
+                "$ref": "#/components/headers/my-header",
+              },
+            },
           },
         },
-      }
-    `);
+      },
+    },
+  },
+}
+`);
   });
 
   it('Supports circular schemas declared in components.schemas', () => {
diff --git a/src/create/schema/metadata.ts b/src/create/schema/metadata.ts
index 59d6277..a32104a 100644
--- a/src/create/schema/metadata.ts
+++ b/src/create/schema/metadata.ts
@@ -1,10 +1,11 @@
+import { isReferenceObject } from '../../openapi';
 import type { oas31 } from '../../openapi3-ts/dist';
 
 export const enhanceWithMetadata = (
   schemaObject: oas31.SchemaObject | oas31.ReferenceObject,
   metadata: oas31.SchemaObject | oas31.ReferenceObject,
 ): oas31.SchemaObject | oas31.ReferenceObject => {
-  if ('$ref' in schemaObject) {
+  if (isReferenceObject(schemaObject)) {
     if (Object.values(metadata).every((val) => val === undefined)) {
       return schemaObject;
     }
diff --git a/src/create/schema/parsers/nullable.test.ts b/src/create/schema/parsers/nullable.test.ts
index 17a020c..98f9a17 100644
--- a/src/create/schema/parsers/nullable.test.ts
+++ b/src/create/schema/parsers/nullable.test.ts
@@ -27,14 +27,8 @@ describe('createNullableSchema', () => {
 
     it('creates an oneOf nullable schema for registered schemas', () => {
       const expected: oas30.SchemaObject = {
-        oneOf: [
-          {
-            $ref: '#/components/schemas/a',
-          },
-          {
-            nullable: true,
-          },
-        ],
+        allOf: [{ $ref: '#/components/schemas/a' }],
+        nullable: true,
       };
       const registered = z.string().openapi({ ref: 'a' });
       const schema = registered.optional().nullable();
@@ -65,10 +59,8 @@ describe('createNullableSchema', () => {
             },
             required: ['b'],
           },
-          {
-            nullable: true,
-          },
         ],
+        nullable: true,
       };
       const schema = z
         .union([z.object({ a: z.string() }), z.object({ b: z.string() })])
@@ -84,15 +76,11 @@ describe('createNullableSchema', () => {
         type: 'object',
         properties: {
           b: {
-            oneOf: [
-              {
-                allOf: [{ $ref: '#/components/schemas/a' }],
-                type: 'object',
-                properties: { b: { type: 'string' } },
-                required: ['b'],
-              },
-              { nullable: true },
-            ],
+            allOf: [{ $ref: '#/components/schemas/a' }],
+            type: 'object',
+            properties: { b: { type: 'string' } },
+            required: ['b'],
+            nullable: true,
           },
         },
         required: ['b'],
diff --git a/src/create/schema/parsers/nullable.ts b/src/create/schema/parsers/nullable.ts
index eeca210..6d50981 100644
--- a/src/create/schema/parsers/nullable.ts
+++ b/src/create/schema/parsers/nullable.ts
@@ -1,6 +1,6 @@
 import type { ZodNullable, ZodTypeAny } from 'zod';
 
-import { satisfiesVersion } from '../../../openapi';
+import { isReferenceObject, satisfiesVersion } from '../../../openapi';
 import type { oas31 } from '../../../openapi3-ts/dist';
 import type { ZodOpenApiVersion } from '../../document';
 import { type SchemaState, createSchemaObject } from '../../schema';
@@ -8,44 +8,53 @@ import { type SchemaState, createSchemaObject } from '../../schema';
 export const createNullableSchema = <T extends ZodTypeAny>(
   zodNullable: ZodNullable<T>,
   state: SchemaState,
-): oas31.SchemaObject => {
+): oas31.SchemaObject | oas31.ReferenceObject => {
   const schemaObject = createSchemaObject(zodNullable.unwrap(), state, [
     'nullable',
   ]);
 
-  if ('$ref' in schemaObject || schemaObject.allOf) {
-    return {
-      oneOf: mapNullOf([schemaObject], state.components.openapi),
-    };
-  }
+  if (satisfiesVersion(state.components.openapi, '3.1.0')) {
+    if (isReferenceObject(schemaObject) || schemaObject.allOf) {
+      return {
+        oneOf: mapNullOf([schemaObject], state.components.openapi),
+      };
+    }
+
+    if (schemaObject.oneOf) {
+      const { oneOf, ...schema } = schemaObject;
+      return {
+        oneOf: mapNullOf(oneOf, state.components.openapi),
+        ...schema,
+      };
+    }
+
+    if (schemaObject.anyOf) {
+      const { anyOf, ...schema } = schemaObject;
+      return {
+        anyOf: mapNullOf(anyOf, state.components.openapi),
+        ...schema,
+      };
+    }
+
+    const { type, ...schema } = schemaObject;
 
-  if (schemaObject.oneOf) {
-    const { oneOf, ...schema } = schemaObject;
     return {
-      oneOf: mapNullOf(oneOf, state.components.openapi),
+      type: mapNullType(type),
       ...schema,
     };
   }
 
-  if (schemaObject.anyOf) {
-    const { anyOf, ...schema } = schemaObject;
+  if (isReferenceObject(schemaObject)) {
     return {
-      anyOf: mapNullOf(anyOf, state.components.openapi),
-      ...schema,
-    };
+      allOf: [schemaObject],
+      nullable: true,
+    } as oas31.SchemaObject;
   }
 
   const { type, ...schema } = schemaObject;
 
-  if (satisfiesVersion(state.components.openapi, '3.1.0')) {
-    return {
-      type: mapNullType(type),
-      ...schema,
-    };
-  }
-
   return {
-    type,
+    ...(type && { type }),
     nullable: true,
     ...schema,
     // https://github.com/OAI/OpenAPI-Specification/blob/main/proposals/2019-10-31-Clarify-Nullable.md#if-a-schema-specifies-nullable-true-and-enum-1-2-3-does-that-schema-allow-null-values-see-1900
diff --git a/src/openapi.ts b/src/openapi.ts
index 79fa89a..03e425f 100644
--- a/src/openapi.ts
+++ b/src/openapi.ts
@@ -1,3 +1,5 @@
+import type { oas31 } from './openapi3-ts/dist';
+
 export const openApiVersions = [
   '3.0.0',
   '3.0.1',
@@ -12,3 +14,8 @@ export const satisfiesVersion = (
   test: OpenApiVersion,
   against: OpenApiVersion,
 ) => openApiVersions.indexOf(test) >= openApiVersions.indexOf(against);
+
+export const isReferenceObject = (
+  schemaOrRef: oas31.SchemaObject | oas31.ReferenceObject,
+): schemaOrRef is oas31.ReferenceObject =>
+  Boolean('$ref' in schemaOrRef && schemaOrRef.$ref);