Combo Box invaid value

When I set a combo box that previously had a valid value in it to an invalid value, it displays he item caption of the previous value. I have tried setting the value to null, actually turned on nullSelectionAllowed, set the value to null, and then turned it off again, with no results. Is there something I am missing or is there a way to get the combo box to not display a item caption when the value is invalid?
Thank you

Hey Freddie, is the ComboBox inside a FieldGroup? Because when you try to commit a FieldGroup with invalid values it’s rolled back to the old value.
What exactly shall be the result? When someone choses an invalid value, the new value shall be ??

No, we are not using FieldGroups, instead we use the “setPropertyDataSource”. Also it is not a case of the user selecting an invalid value, instead we are adding on a new record and when get that new record image it has an invalid value in it, because no value has been assigned yet. I can get the field to clear, but it seems when I set the propertyDataSource of the combo box to the new record image, it displays the item caption for the last record displayed. If I just create a record without displaying one first, it works fine.

What I would like to happen is for the combo box to display a blank item caption in the box, by default.