validwhen returns is required error msg

[Xavier Vanderstukken]

I have the following validation rule :

<field property="newPasswordConfirm"
               depends="validwhen">
               <arg position="0" key="valid.confirmnewpassword"/>
               <var>
                     <var-name>test</var-name>
                     <var-value>(*this* == newPassword)</var-value>
               </var>
       </field>

In my ressource file:
errors.required={0} is required.
errors.invalid={0} is invalid.
valid.confirmnewpassword=Confirm new password

If I enter two differents values for newPasswordConfirm and newPassword the
error message displayed by html:errors tag is :
Confirm new password is required instead of Confirm new password is invalid.

Am I doing something wrong?