Form Validation Error !!

[eluser]Vaibhav132[/eluser]
To validate an url i am using the following callback method

Code:

function validate_url($url){
   $result = @file_get_contents($url);
   if($result == false){
   $this->form_validation->set_message('refurl', 'Enter valid URL');
   return false;
  }
  else{
   return true;
  }
}

If user inserts invalid url ..the following message is thrown

"Unable to access an error message corresponding to your field name."

instead of the one i have set "Enter valid URL".

Is it because i am suppressing file_get_contents function..??