diff --git a/src/Symfony/Component/ClassLoader/DebugClassLoader.php b/src/Symfony/Component/ClassLoader/DebugClassLoader.php
index dbfb5ab1e3..54fe23eabb 100644
--- a/src/Symfony/Component/ClassLoader/DebugClassLoader.php
+++ b/src/Symfony/Component/ClassLoader/DebugClassLoader.php
@@ -82,6 +82,10 @@ class DebugClassLoader
             require $file;
 
             if (!class_exists($class, false) && !interface_exists($class, false) && (!function_exists('trait_exists') || !trait_exists($class, false))) {
+                if (false !== strpos($class, '/')) {
+                    throw new \RuntimeException(sprintf('Trying to autoload a class with an invalid name "%s". Be careful that the namespace separator is "\" in PHP, not "/".'));
+                }
+
                 throw new \RuntimeException(sprintf('The autoloader expected class "%s" to be defined in file "%s". The file was found but the class was not in it, the class name or namespace probably has a typo.', $class, $file));
             }