diff --git a/lib/serializer-utils.js b/lib/serializer-utils.js
index ea7a47d..05f1c26 100644
--- a/lib/serializer-utils.js
+++ b/lib/serializer-utils.js
@@ -41,10 +41,10 @@ module.exports = function (collectionName, record, payload, opts) {
         return item.map(function (val) {
           return String(val);
         });
-      } else {
+      } else if (item) {
         return String(item);
       }
-    } else {
+    } else if (item && item[opts.ref]){
       return String(item[opts.ref]);
     }
   }
@@ -187,13 +187,11 @@ module.exports = function (collectionName, record, payload, opts) {
       if (opts.includedLinks) {
         included.links = getLinks(dest, opts.includedLinks);
       }
-      // only add to included array if the resource is defined
-      if(id !== 'undefined') {
-        pushToIncluded(payload, included);
-      }
+
+      if (typeof id !== 'undefined') { pushToIncluded(payload, included); }
     }
 
-    return id !== 'undefined' ? { type: type, id: id } : null;
+    return typeof id !== 'undefined' ? { type: type, id: id } : null;
   };
 
   this.serializeNested = function (dest, current, attribute, opts) {
diff --git a/test/serializer.js b/test/serializer.js
index dabe24c..b8a2543 100644
--- a/test/serializer.js
+++ b/test/serializer.js
@@ -648,6 +648,28 @@ describe('JSON API Serializer', function () {
     });
   });
 
+  describe('Null relationship', function () {
+    it('should serialize the relationship as { data: null }', function (done) {
+      var dataSet = {
+        id: '54735750e16638ba1eee59cb',
+        firstName: 'Sandro',
+        lastName: 'Munda',
+        address: null,
+      };
+
+      var json = new JSONAPISerializer('user', {
+        attributes: ['firstName', 'lastName', 'address'],
+        address: {
+          ref: 'id',
+          included: false
+        }
+      }).serialize(dataSet);
+
+      expect(json.data.relationships.address).eql({ data: null });
+      done(null, json);
+    });
+  });
+
   describe('Nested of nested document', function () {
     it('should be serialized', function (done) {
       var dataSet = {
@@ -1910,5 +1932,4 @@ describe('JSON API Serializer', function () {
       done(null, json);
     });
   });
-
 });