How to mock Reminder::exists() function in laravel

I have tried to mock Reminder::exists() in Laravel Sentinel function.Please consider following code.

This is input array.

$adminUser = [
    "id"=> 1,
    "name" => "",
    "email" => "sandun.d@gmail.com",
    "password" => "abc123",
    "token" => "1e2b5c4b47955609629214b103b3089da03163f7",
    "app_user_id" => 0,
    "remember_token" => null,
    "status" => 1,
    "created_at" => "2016-05-03 12:12:40",
    "updated_at" => "2016-08-29 19:50:04",
    "first_name" => "Sandunm8",
    "last_name" => "Dissanayake",
    "last_login" => "2016-08-29 19:50:04"
];

This is mocking Reminder facade mocking and call to exits function

$userObject = Cartalyst\Sentinel\Laravel\Facades\Sentinel::shouldReceive('findByCredentials')
    ->once()
    ->withAnyArgs(["email" => "sandun.d@eyepax.com"])
    ->andReturn(json_decode(json_encode($adminUser)));

$reminderObject = [
    "code" => "HOVnwYnlZ54jFNuABj0CoKN7f1dUg4Bc",
    "completed" => 0,
    "user_id" => 32,
    "updated_at" => "2016-09-08 15:01:26",
    "created_at" => "2016-09-08 15:01:26",
    "id" => 34
];

Cartalyst\Sentinel\Laravel\Facades\Reminder::shouldReceive('exists')
    ->once()
    ->with($userObject)
    ->andReturn($reminderObject);

But it is given following error.

Argument 1 passed to Mockery_5_Cartalyst_Sentinel_Reminders_IlluminateReminderRepository::exists() must implement interface Cartalyst\Sentinel\Users\UserInterface, instance of stdClass given, called in C:\xampp\htdocs\lsk\vendor\laravel\framework\src\Illuminate\Support\Facades\Facade.php on line 217 and defined