iam having a problem finding a textbox inside createuserwizard which is inside a contentplaceholder

i have a custom validator for this textbox ..now when i put this customvalidator outside createuser wizard it says "cannot find controltovalidate control id CaptchaValue "

heres is a row inside my createuserwizard

<CreateUserWizard......>

    :
    :
    :
 <tr>

  </td>
  </tr>

    :
    :
    :

</CreateUserWizard>

and i have a customevalidator outside createuserwizard

 <asp:CustomValidator ID="CustomValidator5" runat="server" ControlToValidate="CaptchaValue"
                                                                               ErrorMessage="Image value must match" ValidationGroup="CreateUserWizard1" 
                                                                               EnableClientScript="False"></asp:CustomValidator>