Sharepoint 2010: insert items in a list with BCS fiels

Question

Hi, I have a list with 3 fields in Sharepoint 2010: ID (counter), Description (String) and ProductCode (Business Data). All works fine if I use the user interface and I'm able to choose che ProductCode from the BCS entity. Now I need to write code to insert items in this list. I simply use this:

element["Description"]=var1;
element["ProductCode"]=var2;
element.Update();

The problem is that ProductCode is not recognized as a good value and, if I try to edit the item, che user interface show no value.

Any suggestion?