Undefined index: command in C:wamp64wwwWSMshopproducts.php on line 5

if($_REQUEST['command']=='add' && $_REQUEST['productid']>0){

        $pid=$_REQUEST['productid'];
        addtocart($pid,1);
        header("location:shoppingcart.php");
        exit();
    }

Notice: Undefined index: command in C:\wamp64\www\WSMshop\products.php on line 5
and that's the error, i do what i know can fix it, but nothing happen. Please help me. im still newbie in this. TIA