diff --git a/tests/e2e/optional.spec.ts b/tests/e2e/optional.spec.ts
new file mode 100644
index 00000000..f568158d
--- /dev/null
+++ b/tests/e2e/optional.spec.ts
@@ -0,0 +1,33 @@
+import { Test } from '@nestjs/testing';
+import { AppModule } from '../src/app.module';
+import { ConfigService } from '../../lib';
+
+describe('Optional environment variables', () => {
+  it('should return undefined for optional variables', async () => {
+    const module = await Test.createTestingModule({
+      imports: [AppModule.withValidateFunction(() => ({
+        optional: undefined,
+      }))],
+    }).compile();
+
+    const app = module.createNestApplication();
+    await app.init();
+
+    const optional = module.get(ConfigService).get('optional')
+
+    expect(optional).toEqual(undefined)
+  });
+
+  it('should not assign complex objects back to process.env', async () => {
+    const module = await Test.createTestingModule({
+      imports: [AppModule.withValidateFunction(() => ({
+        complex: {hello: 'there'},
+      }))],
+    }).compile();
+
+    const app = module.createNestApplication();
+    await app.init();
+
+    expect(process.env.complex).toEqual(undefined)
+  });
+});