Undo doesn't undo error on field

[Hersh]

In a "Changed" event I set an error on a field via Errors.Add():

Errors.Add(e.Site, errMsg, errMsg);

On the running form I issue Undo (Ctrl+Z). The original value gets restored
to the field, but the error remains on the field.
I attempted capturing the restoration event in the "Changed" function, so I
could remove the Error programmatically, but it seems that the "Changed"
function doesn't fire on Undo (and e.UndoRedo is never true).

Any suggestions?

Thank you.