diff --git a/src/debug2.cls.php b/src/debug2.cls.php
index 8e528299e..55c0b1e6a 100644
--- a/src/debug2.cls.php
+++ b/src/debug2.cls.php
@@ -89,7 +89,7 @@ public static function FilePath($type)
 		if ($type == 'debug.purge') {
 			$type = 'purge';
 		}
-		$rand = substr(md5(substr(AUTH_KEY, -16)), -16);
+		$rand = defined('AUTH_KEY') ? substr(md5(substr(AUTH_KEY, -16)), -16) : '';
 		return $type . $rand . '.log';
 	}