DataGrid throws NoNullAllowedException

Hi, I have a DataGrid with custom ComboBox and DataTimePicker controls in column styles. When I add a new row, typing values into all columns, then close the form, I get a NoNullAllowedException, with additional information "Column 'RoomType' does not allow nulls". This is one of two columns with custom ComboBox styles. The SelectedValue of the ComboBox is bound to the RoomType column of the dataset. How do I correct this problem? Any ideas? Thanks, Royce