diff --git a/app.yaml b/app.yaml
index 7b3c8b675..a10acad24 100644
--- a/app.yaml
+++ b/app.yaml
@@ -2,7 +2,7 @@
 
 application: drupal
 version: 1
-runtime: php
+runtime: php55
 api_version: 1
 threadsafe: true