diff --git a/core/composer.json b/core/composer.json
index a7a6c6a43a..e1dc246144 100644
--- a/core/composer.json
+++ b/core/composer.json
@@ -170,7 +170,8 @@
     },
     "config": {
         "preferred-install": "dist",
-        "autoloader-suffix": "Drupal8"
+        "autoloader-suffix": "Drupal8",
+        "platform": {"php": "5.5.9"}
     },
     "scripts": {
         "pre-autoload-dump": "Drupal\\Core\\Composer\\Composer::preAutoloadDump",