validation of a checkbox using the "required" rule

[eluser]CIfan1000[/eluser]
If in my controller I have the following validation of a checkbox:

$rules['CheckboxUserAgreement'] = "required";

Then, if the checkbox is not checked, the view containing the checkbox displays:

The User agreement checkbox field must have a value.

I would like it to say: The User agreement checkbox field is required.

Please help.