InvalidOperationException

I'm getting an InvalidOperationException when handling the SelectionChanged
event from a combobox that occurs 'cause the user has modified the
combobox's text. The message of the exception is:

"Cannot Reopen an undo unit while another unit is already open."

How do I find out what other undo units are open?

Failed to include the code that was causing the error. I'm simply applying a
font size to a Run within a RichTextBox FlowDocument object:

EditingCommands.ApplyFontSize.Execute( Convert.ToDouble( val ),
mText.CaretPosition.Parent as IInputElement );



"CSkinner" <cwskinner_no_sppaaam@comcast.net> wrote in message
news:%23SX4KBfBGHA.1180@TK2MSFTNGP09.phx.gbl...
> I'm getting an InvalidOperationException when handling the
> SelectionChanged event from a combobox that occurs 'cause the user has
> modified the combobox's text. The message of the exception is:
>
> "Cannot Reopen an undo unit while another unit is already open."
>
> How do I find out what other undo units are open?
>