diff --git a/src/Handshake/ClientNegotiator.php b/src/Handshake/ClientNegotiator.php
index 547b5ea..62d21e5 100644
--- a/src/Handshake/ClientNegotiator.php
+++ b/src/Handshake/ClientNegotiator.php
@@ -23,7 +23,7 @@ public function __construct(
             ->createRequest('GET', '')
             ->withHeader('Connection'           , 'Upgrade')
             ->withHeader('Upgrade'              , 'websocket')
-            ->withHeader('Sec-WebSocket-Version', $this->getVersion())
+            ->withHeader('Sec-WebSocket-Version', (string)$this->getVersion())
             ->withHeader('User-Agent'           , 'Ratchet');
 
         $perMessageDeflateOptions ??= PermessageDeflateOptions::createDisabled();
diff --git a/src/Handshake/ServerNegotiator.php b/src/Handshake/ServerNegotiator.php
index 94f0549..656c7d9 100644
--- a/src/Handshake/ServerNegotiator.php
+++ b/src/Handshake/ServerNegotiator.php
@@ -79,7 +79,7 @@ public function handshake(RequestInterface $request): ResponseInterface {
         $upgradeResponse = $response
             ->withHeader('Connection'           , 'Upgrade')
             ->withHeader('Upgrade'              , 'websocket')
-            ->withHeader('Sec-WebSocket-Version', $this->getVersionNumber());
+            ->withHeader('Sec-WebSocket-Version', (string)$this->getVersionNumber());
 
         if (count($this->_supportedSubProtocols) > 0) {
             $upgradeResponse = $upgradeResponse->withHeader(