Problem with value DataGridViewComboBoxCell

In VB 2005 Express I have a problem, which completely do not understand. I use a DataGridViewComboBox in a DataGridView. When I change data and save it, I have no problem. But when I close the form I a repeting error, being: System.ArgumentException: DataGridViewComboBoxCell value is not valid.

Can anybody help?

Thanks in advance,
Simon