diff --git a/__tests__/serializers/serializeResourceLinkage.test.ts b/__tests__/serializers/serializeResourceLinkage.test.ts
index e86ecec..efe6f47 100644
--- a/__tests__/serializers/serializeResourceLinkage.test.ts
+++ b/__tests__/serializers/serializeResourceLinkage.test.ts
@@ -18,6 +18,19 @@ describe('`serializeResourceLinkage`', () => {
     chance = new Chance();
   });
 
+  describe('when given a resource linkage', () => {
+    it('should return the resource linkage as is', () => {
+      const resourceLinkage = {
+        type: chance.string(),
+        id: chance.string(),
+      };
+
+      expect(serializeResourceLinkage(resourceLinkage)).toEqual(
+        resourceLinkage,
+      );
+    });
+  });
+
   describe('when given an array of class instances', () => {
     it('should throw an error if some element in the array is not an object', () => {
       const classInstances = [1];
diff --git a/__tests__/serializers/serializeResourceObject.test.ts b/__tests__/serializers/serializeResourceObject.test.ts
index 2eb49cd..1fb7444 100644
--- a/__tests__/serializers/serializeResourceObject.test.ts
+++ b/__tests__/serializers/serializeResourceObject.test.ts
@@ -122,8 +122,8 @@ describe('`serializeResourceObject`', () => {
       });
     });
 
-    describe('when the related class instance is not an object', () => {
-      it('should throw an error', () => {
+    describe('when the related class instance is a single object', () => {
+      it('should throw an error if the related class instance is not an object', () => {
         const key = chance.string();
 
         getMetadataBySymbolMocked.mockImplementation(
@@ -144,9 +144,7 @@ describe('`serializeResourceObject`', () => {
           `Failed to serialize relationship object for ${key} because the value is not an object.`,
         );
       });
-    });
 
-    describe('when the related class instance is an object', () => {
       it('should serialize the relationships object and return it', () => {
         const key = chance.string();
 
@@ -203,49 +201,6 @@ describe('`serializeResourceObject`', () => {
       });
     });
 
-    describe('when the related class instance is already a relationship object', () => {
-      it('should return without serializing the relationship object', () => {
-        const key = chance.string();
-
-        getMetadataBySymbolMocked.mockImplementation(
-          (_: object, symbol: symbol) => {
-            if (symbol === resourceSymbol) {
-              return 'type';
-            }
-
-            if (symbol === relationshipsSymbol) {
-              return [[key, '']];
-            }
-
-            return undefined;
-          },
-        );
-
-        collectMocked.mockImplementation((_: object, symbol: symbol) => {
-          if (symbol === idSymbol) {
-            return 'id';
-          }
-
-          return undefined;
-        });
-
-        const relatedClassInstance = {
-          id: 'some id',
-          type: 'some type',
-        };
-
-        const classInstance = {
-          [key]: {
-            data: relatedClassInstance,
-          },
-        };
-
-        const result = serializeResourceObject(classInstance);
-
-        expect(result.relationships).toBeUndefined();
-      });
-    });
-
     it('should return without the `relationships` field if no relationships are found', () => {
       getMetadataBySymbolMocked.mockImplementation(
         (_: object, symbol: symbol) => {
diff --git a/__tests__/serializers/utils/isRelationshipObject.test.ts b/__tests__/serializers/utils/isRelationshipObject.test.ts
deleted file mode 100644
index ce0864a..0000000
--- a/__tests__/serializers/utils/isRelationshipObject.test.ts
+++ /dev/null
@@ -1,35 +0,0 @@
-import { isRelationshipObject } from '../../../src/serializers/utils/isRelationshipObject';
-import { isResourceLinkage } from '../../../src/serializers/utils/isResourceLinkage';
-
-jest.mock('../../../src/serializers/utils/isResourceLinkage');
-const isResourceLinkageMocked = jest.mocked(isResourceLinkage);
-
-describe('`isRelationshipObject`', () => {
-  it('should return false if `relationshipObjectCandidate` is `null`', () => {
-    const relationshipObjectCandidate = null;
-
-    expect(isRelationshipObject(relationshipObjectCandidate)).toBe(false);
-  });
-
-  it('should return false if `relationshipObjectCandidate` is not an object', () => {
-    const relationshipObjectCandidate = 'not an object';
-
-    expect(isRelationshipObject(relationshipObjectCandidate)).toBe(false);
-  });
-
-  it('should return false if `relationshipObjectCandidate` does not have a `data` property', () => {
-    const relationshipObjectCandidate = {};
-
-    expect(isRelationshipObject(relationshipObjectCandidate)).toBe(false);
-  });
-
-  it('should return true if `relationshipObjectCandidate.data` is a `JSONAPIResourceLinkage`', () => {
-    isResourceLinkageMocked.mockReturnValue(true);
-
-    const relationshipObjectCandidate = {
-      data: 'foo',
-    };
-
-    expect(isRelationshipObject(relationshipObjectCandidate)).toBe(true);
-  });
-});
diff --git a/src/serializers/serializeResourceLinkage.ts b/src/serializers/serializeResourceLinkage.ts
index 671b6f6..9776233 100644
--- a/src/serializers/serializeResourceLinkage.ts
+++ b/src/serializers/serializeResourceLinkage.ts
@@ -3,13 +3,16 @@ import { resourceSymbol } from '../decorators/resource';
 import { collect } from './utils/collect';
 import { getMetadataBySymbol } from './utils/getMetadataBySymbol';
 import { isObject } from './utils/isObject';
+import { isResourceLinkage } from './utils/isResourceLinkage';
 
 import type { JSONAPIResourceLinkage } from '../types/resourceLinkage';
 
 export const serializeResourceLinkage = <I extends object>(
-  classInstanceCandidate_s: I | JSONAPIResourceLinkage,
+  classInstance_s: I | JSONAPIResourceLinkage,
 ): JSONAPIResourceLinkage => {
-  const classInstance_s = classInstanceCandidate_s as I;
+  if (isResourceLinkage(classInstance_s)) {
+    return classInstance_s;
+  }
 
   if (Array.isArray(classInstance_s)) {
     if (!classInstance_s.every(isObject)) {
diff --git a/src/serializers/serializeResourceObject.ts b/src/serializers/serializeResourceObject.ts
index 48bf413..ed034d2 100644
--- a/src/serializers/serializeResourceObject.ts
+++ b/src/serializers/serializeResourceObject.ts
@@ -17,7 +17,6 @@ import type {
   JSONAPIResourceObject,
   JSONObject,
 } from '../types';
-import { isRelationshipObject } from './utils/isRelationshipObject';
 
 export const serializeResourceObject = <I extends object>(
   classInstance: I,
@@ -44,10 +43,6 @@ export const serializeResourceObject = <I extends object>(
       );
     }
 
-    if (isRelationshipObject(relatedClassInstance_s)) {
-      return acc;
-    }
-
     acc[key.toString()] = {
       data: serializeResourceLinkage(relatedClassInstance_s),
     };
diff --git a/src/serializers/utils/index.ts b/src/serializers/utils/index.ts
index 57072d2..67ffb17 100644
--- a/src/serializers/utils/index.ts
+++ b/src/serializers/utils/index.ts
@@ -3,6 +3,5 @@ export * from './getMetadataBySymbol';
 export * from './collect';
 export * from './clean';
 export * from './isObject';
-export * from './isRelationshipObject';
 export * from './isResourceIdentifierObject';
 export * from './isResourceLinkage';
diff --git a/src/serializers/utils/isRelationshipObject.ts b/src/serializers/utils/isRelationshipObject.ts
deleted file mode 100644
index 89b78f8..0000000
--- a/src/serializers/utils/isRelationshipObject.ts
+++ /dev/null
@@ -1,23 +0,0 @@
-import { isResourceLinkage } from './isResourceLinkage';
-
-import type { JSONAPIRelationshipObject } from '../../types/relationshipObject';
-
-export const isRelationshipObject = (
-  relationshipObjectCandidate: unknown,
-): relationshipObjectCandidate is JSONAPIRelationshipObject => {
-  if (relationshipObjectCandidate === null) {
-    return false;
-  }
-
-  if (typeof relationshipObjectCandidate !== 'object') {
-    return false;
-  }
-
-  if (!('data' in relationshipObjectCandidate)) {
-    return false;
-  }
-
-  const { data } = relationshipObjectCandidate;
-
-  return isResourceLinkage(data);
-};