ComboxBox error?

Question

I have linked a ComboBox to a DataView and set its DisplayMember and ValueMember. I have filled the DataTable from which the DataView is derived and set the RowFilter on the DataView. Ok. Now I pick a row in the ComboBox. All is okay. But if I now set the SelectedValue = Convert.DbNull (or SelectedIndex=-1) the control "overrides" this and changes SelectedIndex=0 and SelectedValue to the first row of the view. If I set SelectedValue=Convert.DbNull a second time, I get what I expect: SelectedIndex=-1. Any ideas why it behaves this way? Is there some other way to indicate you want to reset the ComboBox so no row is selected (and the displayed text is blank)?