custom validation returns error always

I have a custom validator that returns error always.

class myValidation extends CI_Form_validation {
/**
* myValidation::
*/

function fieldvalueisok($value){
//validates value against illegal entries such as "xxx"

$bannedValues=array('a','b','c');
if(in_array(strtolower($value),$bannedValues)){
return false;
}
return true;
}

}

Even if i return true everywhere, in a blank function etc, still i get an error from it. What am I missing? Please i wanna know more than the method above..the above is just an eg.