form validation - default msg showing instead of my own

[eluser]stef25[/eluser]
I decided to write my own validation function, but neither of these echo messages seem to come out. the checkbox markup:

Code:

<input type="checkbox" class="checkbox" id="chk_approval" value="accept" name="chk_approval"/>

Then my validation function:

Code:

$rules['chk_approval'] = "callback_checkbox_check";
                
                //custom validation function for the checkbox
                function checkbox_check() {
                    $value = "accept";
                    if($this->post->chk_approval == $value) {
                        echo "OK!";
                    }                        
                    else {
                        echo "not OK!";
                    }

Is this code wrong?