Vista - Binding to custom class, no updates back to UI

Hi,

I have created a usercontrol with some textboxes.
A textbox (text-property) is binding to a customer-class (some property).
The customer-class is implementing INotifyPropertyChanged, and the properties raise the events on change.
Everything is working fine, i change the textbox, the property on the customer is changed too.

My problem:
I have some logic in the customer-class.
If the property is assigned something illegal, the value is rejected. And maybe some default-value is set.
That new value isn't send back to the UI (textbox), not even if i raise the PropertyChanged-event.

Is there something i forgot to implement?

Debbus