Laravel 5.2 - Use a String as a Custom Primary Key for Eloquent Table becomes 0

I am trying to use email as my table's primary key, so my eloquent code is-

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class UserVerification extends Model
{
    protected $table = 'user_verification';
    protected $fillable =   [
                                'email',
                                'verification_token'
                            ];
    //$timestamps = false;
    protected $primaryKey = 'verification_token';
}

And my DB is like this-

but if I do this-

UserVerification::where('verification_token', $token)->first();

I am getting this-

{"email":"sdfsdf@sdfsdf.sdf","verification_token":0,"created_at":"2016-01-03 22:27:44","updated_at":"2016-01-03 22:27:44"}

So, the verification token/primary key becomes 0.

Can anyone please help?

via Chebli Mohamed