RequiredFieldValidator and updatepanel error

hello to all
i have 3texbox,1button for change password (use asp.net membership change password) and use RequiredFieldValidator in updatepanel,
The first when i fill textboxs and click in the button, allthing is good,
but for the second time when i fill texbox and click in the button all texbox is empty and RequiredFieldValidator is fire and get error for fill texbox 
 
To solve this problem, what should I do?
 
thank's