diff --git a/src/Symfony/Component/Routing/RouteCollection.php b/src/Symfony/Component/Routing/RouteCollection.php
index 359c0bfee0..b90c7cb90f 100644
--- a/src/Symfony/Component/Routing/RouteCollection.php
+++ b/src/Symfony/Component/Routing/RouteCollection.php
@@ -120,10 +120,18 @@ class RouteCollection implements \IteratorAggregate
      */
     public function addPrefix($prefix)
     {
+        // a prefix must not end with a slash
+        $prefix = rtrim($prefix, '/');
+
         if (!$prefix) {
             return;
         }
 
+        // a prefix must start with a slash
+        if ('/' !== $prefix[0]) {
+            $prefix = '/'.$prefix;
+        }
+
         $this->prefix = $prefix.$this->prefix;
 
         foreach ($this->all() as $route) {