Proof-of-Pass Receipt

diff --git a/src/__tests__/factory.test.ts b/src/__tests__/factory.test.ts
index 1c5b26dd27c11a1f6b57499daaa9708d9c603afe..49dcf97eb41607fe6381b06417107d9d522dd4eb 100644
--- a/src/__tests__/factory.test.ts
+++ b/src/__tests__/factory.test.ts
@@ -1,6 +1,30 @@
 import { beforeEach, describe, expect, it } from 'vitest';
 import { createFactory } from '../factory.js';
 
+describe('createFactory', () => {
+  it('throws when the factory-builder callback returns undefined', () => {
+    const invalidFactoryBuilderCallback = () => undefined;
+
+    expect(() =>
+      // @ts-expect-error - exercise invalid runtime callback behavior
+      createFactory(invalidFactoryBuilderCallback)
+    ).toThrowError(
+      'The factory-builder callback must return the supplied factory builder.'
+    );
+  });
+
+  it('throws when the factory-builder callback returns a different value', () => {
+    const invalidFactoryBuilderCallback = () => ({});
+
+    expect(() =>
+      // @ts-expect-error - exercise invalid runtime callback behavior
+      createFactory(invalidFactoryBuilderCallback)
+    ).toThrowError(
+      'The factory-builder callback must return the supplied factory builder.'
+    );
+  });
+});
+
 describe('build', () => {
   type User = {
     id: number;
diff --git a/src/factory.ts b/src/factory.ts
index b7d9c15122cc62878d8c3d9b52ac561d5a2eff10..f8add5b2a39520fb72293f5a72f7a907a10a1e96 100644
--- a/src/factory.ts
+++ b/src/factory.ts
@@ -296,7 +296,12 @@ export function createFactory<
 
   if (typeof attributesOrBuilder === 'function') {
     const factoryBuilder = createFactoryBuilder(definition);
-    attributesOrBuilder(factoryBuilder);
+    const returnedFactoryBuilder = attributesOrBuilder(factoryBuilder);
+    if (returnedFactoryBuilder !== factoryBuilder) {
+      throw new Error(
+        'The factory-builder callback must return the supplied factory builder.'
+      );
+    }
   } else {
     definition.attributeDefaults = attributesOrBuilder;
   }