diff --git a/tests/Unit/ListenForLdapBindFailureTest.php b/tests/Unit/ListenForLdapBindFailureTest.php
index 0336894..c25fdfe 100644
--- a/tests/Unit/ListenForLdapBindFailureTest.php
+++ b/tests/Unit/ListenForLdapBindFailureTest.php
@@ -41,6 +41,7 @@ public function test_validation_exception_is_not_thrown_when_invalid_credentials
 
         /** @var \LdapRecord\Testing\LdapFake $ldap */
         $ldap = $fake->getLdapConnection();
+        $ldap->shouldReturnDiagnosticMessage(null);
         $ldap->shouldReturnError('Invalid credentials');
 
         $this->assertFalse($fake->auth()->attempt('user', 'secret'));