Laravel Sum of relation

I want to return the sum of "amount" from my payments table. There can be many payments for one invoice. The below "->sum('amount') does not work, it returns:

Call to a member function addEagerConstraints() on a non-object.

How to return the sum of all payments for each invoice in my relation?

Invoices Model:

class Invoices extends Eloquent {

    public function payments()
    {
        return $this->hasMany('Payments')->sum('amount');
    }
}

Expenses Model:

class Payments extends Eloquent {

    public function invoices()
    {
        return $this->belongsTo('Invoices');
    } 
}

My table "payments" holds the foreign key of my tables invoices, which is invoices_id.

class Invoices extends Eloquent {

    public function payments()
    {
        return $this->hasMany('Payments');
    }
}

class Payments extends Eloquent {

    public function invoices()
    {
        return $this->belongsTo('Invoices');
    } 
}

In your controller

Invoice::with(['payments' => function($query){
   $query->sum('amount');
}])->get();

;