diff --git a/src/Symfony/Component/ClassLoader/ClassCollectionLoader.php b/src/Symfony/Component/ClassLoader/ClassCollectionLoader.php
index 5d09848952..6f88286de0 100644
--- a/src/Symfony/Component/ClassLoader/ClassCollectionLoader.php
+++ b/src/Symfony/Component/ClassLoader/ClassCollectionLoader.php
@@ -275,7 +275,13 @@ REGEX;
      */
     private static function writeCacheFile($file, $content)
     {
-        $tmpFile = tempnam(dirname($file), basename($file));
+        $dir = dirname($file);
+        if (!is_writable($dir)) {
+            throw new \RuntimeException(sprintf('Cache directory "%s" is not writable.', $dir));
+        }
+
+        $tmpFile = tempnam($dir, basename($file));
+
         if (false !== @file_put_contents($tmpFile, $content) && @rename($tmpFile, $file)) {
             @chmod($file, 0666 & ~umask());