jwtAuth login with laravel 5.2 on custom field

I'm trying to implement token based authentication in laravel 5.2. I'm following this blog to implement. After doing all the things, when I'm going to test, I got 500 error. On debugging the code, I found that jwt calls a function (once($credentials)) in vendor/tymon/jwt-auth/src/Providers/Auth/IlluminateAuthAdapter.php file. And once() function is not defined in any file of jwt-auth package. If anyone have faced same problem before and knows the solution, it will be appreciated.

via Chebli Mohamed