diff --git a/rules/__tests__/prefer_to_be_undefined.test.js b/rules/__tests__/prefer_to_be_undefined.test.js
index 63d129518..a4d574b88 100644
--- a/rules/__tests__/prefer_to_be_undefined.test.js
+++ b/rules/__tests__/prefer_to_be_undefined.test.js
@@ -6,7 +6,10 @@ const rules = require('../../').rules;
 const ruleTester = new RuleTester();
 
 ruleTester.run('prefer_to_be_undefined', rules['prefer-to-be-undefined'], {
-  valid: ['expect(undefined).toBeUndefined();'],
+  valid: [
+    'expect(undefined).toBeUndefined();',
+    'expect(undefined).not.toBeUndefined();',
+  ],
 
   invalid: [
     {