Extending FormHelper causes error in it

I want to extend the FormHelper and used this part of a method's code to be called in the view.
$out = parent::input($fieldName, $opts);
return parent::output($out);

Error: Call to a member function useTag() on a non-object 
File: ...lib/Cake/View/Helper/FormHelper.php 
Line: 861

the label method 
return $this->Html->useTag('label', $labelFor, $options, $text);

cakephp version 2.5.1

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