diff --git a/src/collapsed.js b/src/collapsed.js
index 408e7cc..9d47071 100644
--- a/src/collapsed.js
+++ b/src/collapsed.js
@@ -21,11 +21,21 @@ export default function inspectCollapsed(object, shallow, name, proto) {
   let tag, fields, next, n;
 
   if (object instanceof Map) {
-    tag = `Map(${object.size})`;
-    fields = iterateMap;
+    if (object instanceof object.constructor) {
+      tag = `Map(${object.size})`;
+      fields = iterateMap;
+    } else { // avoid incompatible receiver error for prototype
+      tag = "Map()";
+      fields = iterateObject;
+    }
   } else if (object instanceof Set) {
-    tag = `Set(${object.size})`;
-    fields = iterateSet;
+    if (object instanceof object.constructor) {
+      tag = `Set(${object.size})`;
+      fields = iterateSet;
+    } else { // avoid incompatible receiver error for prototype
+      tag = "Set()";
+      fields = iterateObject;
+    }
   } else if (arrayish) {
     tag = `${object.constructor.name}(${object.length})`;
     fields = iterateArray;
diff --git a/src/expanded.js b/src/expanded.js
index 756c5db..a04f7db 100644
--- a/src/expanded.js
+++ b/src/expanded.js
@@ -15,11 +15,21 @@ export default function inspectExpanded(object, _, name, proto) {
   let tag, fields, next, n;
 
   if (object instanceof Map) {
-    tag = `Map(${object.size})`;
-    fields = iterateMap;
+    if (object instanceof object.constructor) {
+      tag = `Map(${object.size})`;
+      fields = iterateMap;
+    } else { // avoid incompatible receiver error for prototype
+      tag = "Map()";
+      fields = iterateObject;
+    }
   } else if (object instanceof Set) {
-    tag = `Set(${object.size})`;
-    fields = iterateSet;
+    if (object instanceof object.constructor) {
+      tag = `Set(${object.size})`;
+      fields = iterateSet;
+    } else { // avoid incompatible receiver error for prototype
+      tag = "Set()";
+      fields = iterateObject;
+    }
   } else if (arrayish) {
     tag = `${object.constructor.name}(${object.length})`;
     fields = iterateArray;
diff --git a/test/__snapshots__/inspector.test.js.snap b/test/__snapshots__/inspector.test.js.snap
index cd62978..a4faa55 100644
--- a/test/__snapshots__/inspector.test.js.snap
+++ b/test/__snapshots__/inspector.test.js.snap
@@ -774,3 +774,65 @@ hi
   </div>
 </div>
 `;
+
+exports[`map and set proto map proto 1`] = `
+<div>
+  <div
+    class="observablehq"
+  >
+    <span
+      class="observablehq--collapsed observablehq--inspect"
+    >
+      <a>
+        <svg
+          class="observablehq--caret"
+          height="8"
+          width="8"
+        >
+          
+    
+          <path
+            d="M7 4L1 8V0z"
+            fill="currentColor"
+          />
+          
+  
+        </svg>
+        Map() {
+      </a>
+      }
+    </span>
+  </div>
+</div>
+`;
+
+exports[`map and set proto set proto 1`] = `
+<div>
+  <div
+    class="observablehq"
+  >
+    <span
+      class="observablehq--collapsed observablehq--inspect"
+    >
+      <a>
+        <svg
+          class="observablehq--caret"
+          height="8"
+          width="8"
+        >
+          
+    
+          <path
+            d="M7 4L1 8V0z"
+            fill="currentColor"
+          />
+          
+  
+        </svg>
+        Set() {
+      </a>
+      }
+    </span>
+  </div>
+</div>
+`;
diff --git a/test/inspector.test.js b/test/inspector.test.js
index 9b453e6..98c152a 100644
--- a/test/inspector.test.js
+++ b/test/inspector.test.js
@@ -98,6 +98,23 @@ describe("into", () => {
   });
 });
 
+describe("map and set proto", () => {
+  test("map proto", () => {
+    class SubMap extends Map {}
+    const container = document.createElement("div");
+    const inspector = Inspector.into(container)();
+    inspector.fulfilled(Object.getPrototypeOf(new SubMap()));
+    expect(container).toMatchSnapshot();
+  });
+  test("set proto", () => {
+    class SubSet extends Set {}
+    const container = document.createElement("div");
+    const inspector = Inspector.into(container)();
+    inspector.fulfilled(Object.getPrototypeOf(new SubSet()));
+    expect(container).toMatchSnapshot();
+  });
+});
+
 describe("immutable", () => {
   let inspector, elem;
   beforeEach(() => {