Custom Zend Error message for checkboxes

I have a form in a Zend-based site which has a required "Terms and Conditions" checkbox.

I have set a custom message which says "you must agree with terms and conditions".

however, because the checkbox is "presence='required'", it returns

Field 'terms' is required by rule 'terms', but the field is missing

which is this constant defined in the Zend framework:

self::MISSING_MESSAGE     => "Field '%field%' is required by rule '%rule%', but the field is missing",

I could edit this constant, but this would change the error reporting for all required checkboxes.

How can I affect the error reporting for this specific case?

From stackoverflow

• You can override the default message like this:

  $options = array(
                  'missingMessage' => "Field '%field%' is required by rule '%rule%', dawg!"
              );
  

  And then:

  $input = new Zend_Filter_Input($filters, $validators, $myData);
  

  Or

  $input = new Zend_Filter_Input($filters, $validators, $myData);
  $input->setOptions($options);
  

  ...and finally:

  if ($input->hasInvalid() || $input->hasMissing()) {
    $messages = $input->getMessages();
  }
  

  It's mentioned on the Zend_Filter_Input manual page.

• If you are using the Zend_Form_Element_Checkbox you can customize the error messages on the Zend_Validate validators.

  $form->addElement('checkbox', 'terms', array(
    'label'=>'Terms and Services',
    'uncheckedValue'=> '',
    'checkedValue' => 'I Agree',
    'validators' => array(
      // array($validator, $breakOnChainFailure, $options)
      array('notEmpty', true, array(
        'messages' => array(
          'isEmpty'=>'You must agree to the terms'
        )
      ))
     ),
     'required'=>true,
  );
  

  You want to make sure the unchecked value is "blank" and that the field is "required"