Unable to prepare route [api/user] for serialization. Uses Closure.

Hi All,

Has anyone come across the below error when running php artisan route:cache

  LogicException  : Unable to prepare route [api/user] for serialization. Uses Closure.

  at /home/vagrant/code/project4/vendor/laravel/framework/src/Illuminate/Routing/Route.php:917
    913|      */
    914|     public function prepareForSerialization()
    915|     {
    916|         if ($this->action['uses'] instanceof Closure) {
  > 917|             throw new LogicException("Unable to prepare route [{$this->uri}] for serialization. Uses Closure.");
    918|         }
    919|
    920|         $this->compileRoute();
    921|

I havent made a change to the api/user route which is as below:

Route::middleware('auth:api')->get('/user', function (Request $request) {
    return $request->user();
});