I can't tell from your description exactly what is going on (i.e. why the error is occurring) but to answer your specific question - the InvalidValueBehavior property of the event arguments determines what action is taken. This property defaults based on the resolved InvalidValueBehavior property which ultimately defaults to DisplayErrorMessage. You should set this property on the event arguments of that event to one of the other values if you do not want to display an error message. You can also set the ErrorMessage property to null or a 0 length string to avoid the message box.