diff --git modules/user/user.test modules/user/user.test
index 0c184b8..46c6623 100644
--- modules/user/user.test
+++ modules/user/user.test
@@ -1315,6 +1315,10 @@ class UserSaveTestCase extends DrupalWebTestCase {
 
     $user_by_name = user_load_by_name($test_name);
     $this->assertTrue($user_by_name, t('Loading user by name.'));
+
+    // Mimic a broken module calling user_save with an empty key.
+    user_save($user_by_name, array('' => $this->randomName()));
+    $this->drupalGet('user/' . $user_by_name->uid);
   }
 }