Follow-up to #1398, #1548
diff --git a/index.php b/index.php
index c379f72..aa1df44 100644
--- a/index.php
+++ b/index.php
@@ -66,6 +66,7 @@
 	break;
 
 	default:
+		header('HTTP/1.1 503 Service Unavailable.', TRUE, 503);
 		exit('The application environment is not set correctly.');
 }