Procedure returning different values

I have a procedure TempSalesUpdateID which gets all ProductName's column in TempSales table and set it's corresponding ProductID. ALTER PROCEDURE [dbo].[TempSalesUpdateID] AS UPDATE T SET T.ProductID = P.ProductID FROM TempSales T INNER JOIN Products P ON P.PN=T.ProductName When I execute it through SQL Server Management Studio it runs perfectly: when it finds a corresponding ID, it fills ProductID column, otherwise leaves it NULL. When I run it from my C# Windows Form Application, it puts almost random numbers instead of leaving NULL (the ones which doesn't have a corresponding ID). I say almost because: 13098 rows have value 1 instead of NULL, 14742 rows have value 2 instead of NULL, 13601 value 3, etc. Could you please help me? I'm really lost here