Call to undefined function App\belongsToMany [Laravel]

The error i am getting is "Call to undefined function App\belongsToMany".

This is one of the two models that is used for the relationship:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Review extends Model
{
protected $table = "reviews";

protected $fillable = [ 'user_id','email','review'];

public function user()
{
    return $this->belongsTo('App\User');
}

public function votes()
{
    return $this->belongsToMany('App\User')->withPivot('vote')->withTimestamps();
}

public function categories()
{
    return $this-belongsToMany('Category','category_review','review_id','category_id')->withTimestamps();
}

public function tags()
{
    return $this->belongsToMany('App\Tag')->withTimestamps();
}

}

My other model :

<?php

 namespace App;

 use Illuminate\Database\Eloquent\Model;

 class Category extends Model
 {
public function reviews()
{
    return $this->belongsToMany('App\Review','category_review','category_id','review_id')->withTimestamps();
}

public function children()
{
    return $this->hasMany('App\Category','parent_id');
}

public function parent()
{
    return $this->belongsTo('App\Category','parent_id');
}

}

The problem is, i can run the App\Category::find(1)->reviews; but, i can't run App\Review::find(1)->categories; It says "Call to undefined function App\BelongsToMany"

via Chebli Mohamed