Call to a member function addEagerConstraints() on string

I want to calculate the average of the column of nested morph relation.

Model Function

public function trader_ratings()
{
    return $this->morphMany(TraderRatings::class, 'rateable')->select('rateable_id','rateable_type','rating')->avg('rating');
}

Controller with lazy loading

            $user = auth('api')->user();
            $user_id = $user->id;
            $customer_classes = CustomerClassBooking::with([
             'trader_class',
             'trader_class.trader_ratings'

            ,'vendor'])

            ->where('customer_id',$user_id)
            ->where('status','Accepted-paid')
            ->get();

It is not calculating the avg but giving the error

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