diff --git a/lib/JSONAPISerializer.js b/lib/JSONAPISerializer.js
index 2bb24ba..492e292 100644
--- a/lib/JSONAPISerializer.js
+++ b/lib/JSONAPISerializer.js
@@ -2,7 +2,7 @@ const {
   pick,
   isEmpty,
   omit,
-  isPlainObject,
+  omitBy,
   transform,
   get,
   set,
@@ -678,15 +678,18 @@ module.exports = class JSONAPISerializer {
     const serializedRelationship = { type };
 
     // Support for unpopulated relationships (an id, or array of ids)
-    if (!isPlainObject(rData)) {
+    if (typeof rData !== 'object') {
       serializedRelationship.id = rData.toString();
     } else {
       // Relationship has been populated
       serializedRelationship.id = rData[rOptions.id].toString();
-      included.set(
-        `${type}-${serializedRelationship.id}`,
-        this.serializeResource(type, rData, rOptions, included, extraData)
-      );
+      const filteredKeys = Object.keys(omitBy(rData, x => !x));
+      if (!(filteredKeys.length === 1 && filteredKeys[0] === 'id')) {
+        included.set(
+          `${type}-${serializedRelationship.id}`,
+          this.serializeResource(type, rData, rOptions, included, extraData)
+        );
+      }
     }
     return serializedRelationship;
   }
@@ -746,7 +749,7 @@ module.exports = class JSONAPISerializer {
       converted = transform(
         data,
         (result, value, key) => {
-          if (typeof value === 'object') {
+          if (value && typeof value === 'object') {
             result[this._convertCase(key, convertCaseOptions)] = this._convertCase(
               value,
               convertCaseOptions
diff --git a/lib/helpers.js b/lib/helpers.js
index 7fe6dea..8c0e9ac 100644
--- a/lib/helpers.js
+++ b/lib/helpers.js
@@ -2,6 +2,7 @@ const {
   pick,
   isEmpty,
   omit,
+  omitBy,
   isPlainObject,
   transform,
   toKebabCase,
@@ -23,6 +24,7 @@ module.exports = {
   pick,
   isEmpty,
   omit,
+  omitBy,
   isPlainObject,
   transform,
   toKebabCase,