captcha validation & fd:failmessage

[321los]

Hello,

my aim is to use captcha validation on forms, including a custom failmessage.
My try was like that:


<fd:captcha id="capt" required="true" length="5">
      <fd:help>case sensitive!</fd:help>
      <fd:datatype base="string"/>
      <fd:validation>
        <fd:captcha>
            <fd:failmessage>Wrong! Try again</fd:failmessage>
        </fd:captcha>
      </fd:validation>
</fd:captcha>


but the displayed error message still is "validation.captcha.mismatch"
Whats my mistake?

Thanks for your help
Marco