error Trying to get property of non-object in Laravel 5 with relation and scope

I'm working in Laravel 5. My situation is the next: I have a table USERS and a table PROFILES the have a relation 1:1. So I have in the model users a method called profile like this:

public function profile(){      
        return $this->belongsTo('App\models\Profile','profile_id');
    }

Also I have a method scopeData where I want to filter what users have identity_doc equal as data:

public function scopeData($query,$data){
        $profile=$this->profile;
        $query->where($profile->identity_doc,$data);
    }

in the user controller I use the scope function in this line:

$students= Users::data($request->get('data'))->where('rol','=','Student')->get();

but, when I runned it I have this error: Trying to get property of non-object so, why it happens? the profile method works fine in others functions, but no here, I get:

**array('query' => object(Builder), 'data' => 'example', 'profile' => null))**

via Chebli Mohamed