diff --git a/src/Mock/OpenApiDataMockerRouteMiddleware.php b/src/Mock/OpenApiDataMockerRouteMiddleware.php
index 0646fd2..498593e 100644
--- a/src/Mock/OpenApiDataMockerRouteMiddleware.php
+++ b/src/Mock/OpenApiDataMockerRouteMiddleware.php
@@ -87,7 +87,7 @@ public function process(ServerRequestInterface $request, RequestHandlerInterface
         $customCallback = $this->getMockStatusCodeCallback;
         $customAfterCallback = $this->afterCallback;
         $mockedStatusCode = (is_callable($customCallback)) ? $customCallback($request, $this->responses) : null;
-        if (array_key_exists($mockedStatusCode, $this->responses)) {
+        if ($mockedStatusCode && array_key_exists($mockedStatusCode, $this->responses)) {
             // response schema successfully selected, we can mock it now
             $statusCode = ($mockedStatusCode === 'default') ? 200 : (int) $mockedStatusCode;
             $mockedResponse = (array) $this->responses[$mockedStatusCode];