Add laravie/api@51f9c85 as suggested by @crynobone .

Lumen stable isn't 5.3 yet. Thanks @hskrasek

composer.json

@@ -31,7 +31,7 @@
         "illuminate/filesystem": "5.1.* || 5.2.* || 5.3.*",
         "illuminate/log": "5.1.* || 5.2.* || 5.3.*",
         "illuminate/pagination": "5.1.* || 5.2.* || 5.3.*",
-        "laravel/lumen-framework": "5.1.* || 5.2.* || 5.3.*",
+        "laravel/lumen-framework": "5.1.* || 5.2.*",
         "lucadegasperi/oauth2-server-laravel": "5.0.*",
         "mockery/mockery": "~0.9",
         "phpunit/phpunit": "^4.8 || ^5.0",

src/Routing/Adapter/Laravel.php

@@ -226,6 +226,10 @@ public function prepareRouteForSerialization($route)
      */
     public function gatherRouteMiddlewares($route)
     {
+        if (method_exists($this->router, 'gatherRouteMiddleware')) {
+            return $this->router->gatherRouteMiddleware($route);
+        }
+
         return $this->router->gatherRouteMiddlewares($route);
     }