diff --git a/Version/RFC6455.php b/Version/RFC6455.php
index 94416c3..aab0435 100644
--- a/Version/RFC6455.php
+++ b/Version/RFC6455.php
@@ -50,7 +50,7 @@ class RFC6455 implements VersionInterface {
      * {@inheritdoc}
      */
     public function isProtocol(RequestInterface $request) {
-        $version = (int)$request->getHeader('Sec-WebSocket-Version', -1);
+        $version = (int)((string)($request->getHeader('Sec-WebSocket-Version', -1)));
 
         return ($this->getVersionNumber() === $version);
     }
@@ -268,4 +268,4 @@ class RFC6455 implements VersionInterface {
         $this->closeCodes[Frame::CLOSE_SRV_ERR]     = true;
         //$this->closeCodes[Frame::CLOSE_TLS]         = true;
     }
-}
\ No newline at end of file
+}