diff --git a/tests/test_framework/phpbb_functional_test_case.php b/tests/test_framework/phpbb_functional_test_case.php
index 684d7a84cb..66af0db2b1 100644
--- a/tests/test_framework/phpbb_functional_test_case.php
+++ b/tests/test_framework/phpbb_functional_test_case.php
@@ -577,6 +577,7 @@ class phpbb_functional_test_case extends phpbb_test_case
 
 		// Any output before the doc type means there was an error
 		$content = self::$client->getResponse()->getContent();
+		self::assertNotContains('[phpBB Debug]', $content);
 		self::assertStringStartsWith('<!DOCTYPE', trim($content), 'Output found before DOCTYPE specification.');
 	}