I am trying to merge two queries

$sales = DB::table('sales')
    ->join('drugs', 'drugs.id', '=', 'sales.drug_id')
    ->select('sales.*','drugs.name', DB::raw('sum(sales.quantity_sold) as total_sales'))
    ->whereYear('date_sold','>','2014')
    ->whereYear('date_sold','<','2017')
     ->get()->groupBy('sales.drug_id');

$stocks = DB::table('sales')
     ->join('drugs', 'drugs.id', '=', 'sales.drug_id')
     ->select('sales.*','drugs.name', DB::raw ('AVG(sales.quantity_sold) as average_sales'))
     ->whereMonth('date_sold','=', Carbon::today()->month)
      ->get()->groupBy('sales.drug_id');

$data = $stocks->merge($sales);

I get an error, Call to a member function groupBy() on a non-object. Any help?