Parameter Value For Combo Box Needed During Form_close

Hello
Once more I'm desperate:
The most "stupid" thing happens when I close a form of mine:
A message box appears asking me to give a parameter value for a combo box that is included in my form. That combo box is used to find specific records in my form. It works perfectly, but the only problem that creates is that mentioned above. Why is a parameter value needed (only when I close the form)? The funny thing is that if I load the form and not click or update the combo-box, when I close the form everything works fine. I checked the vb-code a million times and there's nothing odd in the "after update" code. I must mention that when that msg box appears, whether i give a value to that message box or not,the form closes normally, without anything affected. But it's very annoying. If we could just by-pass that msg box, or eliminate the cause that makes it appear.......
Pls help.....
Thank you in advance