'empty_data = true' to set default value of checkbox gives error.  

I wanted to set the default value of a checkbox to checked in the following way:
<code><pre>$form->add('public', 'checkbox', array (
    'label' => 'Registration is public',
    'required' => false,
     'empty_data'=>true 
)); 
</pre></code>

but when i uncheck the box, it gives me an error: "This value is not valid."

Is this a bug?
