Authentication user provider is not defined in laravel

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i will like to implement custom authentication on my app for the admin and users section but the whole concept is confusing maybe it is new to me in laravel 5.2 (my version) but at this stage i can say if i am getting it right or wrong but this is the error that is displaying.

InvalidArgumentException in CreatesUserProviders.php line 40:
Authentication user provider [] is not defined.

so i have done what @imrealashu answered but still i have issues.

via Chebli Mohamed