cara menghitung "nama barang" muncul berapa kali sesuai custid

saya membuat syntax seperti ini :

alter FUNCTION Production.TotalPembelian (@custid INT)
 RETURNS TABLE AS
 RETURN
	SELECT custid AS ' Id Produk', pro.productid  AS ' Id Produk',
	FROM Production.Products as pro

	JOIN Sales.OrderDetails as sup
	on pro.productid = sup.productid

	JOIN Sales.Orders as ord
	on sup.orderid = ord.orderid
WHERE custid = @custid
GROUP BY custid,pro.productname
--GROUP BY pro.productname
--ORDER BY custid
GO

tapi terjadi error :

Column 'Production.Products.productid' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

adakah yang tahu letak kesalahannya?