diff --git a/lib/Doctrine/ORM/Proxy/ProxyFactory.php b/lib/Doctrine/ORM/Proxy/ProxyFactory.php
index 94da9a7d9..8051bc39a 100644
--- a/lib/Doctrine/ORM/Proxy/ProxyFactory.php
+++ b/lib/Doctrine/ORM/Proxy/ProxyFactory.php
@@ -144,7 +144,7 @@ class ProxyFactory
 
         $file = str_replace($placeholders, $replacements, $file);
 
-        file_put_contents($fileName, $file);
+        file_put_contents($fileName, $file, LOCK_EX);
     }
 
     /**