I have a zend form that I'm initializing as follows:

$form = new Form_XYZ();

I have a display group in that form, which I'm calling like this:

    $form->addDisplayGroup($generalSettingsGroup,
                           'general',
                            array(
                                'legend' => 'General',
                                'disableDefaultDecorators' => true,
                                'decorators' => array(
                                                    'FormElements',
                                                    'FieldSet',
                                                     array('HtmlTag', 
                                                            array('tag' => 'div', 'class' => 'general')
                                                           )
                                                    )
                                )
                            );

I get this error :

    ArrayObject Object
    (
         [exception] => Zend_Loader_PluginLoader_Exception Object
         (
             [message:protected] => Plugin by name 'FieldSet' was not found in the registry;   used paths:Zend_Form_Decorator_: Zend/Form/Decorator/
             [string:private] => 
             [code:protected] => 0
             [file:protected] => /usr/share/pear/PEAR/Zend/Loader/PluginLoader.php
             [line:protected] => 406
             [trace:private] => Array
             ...................

How can I add the FieldSet plugin to the registry? I'm using the default Zend_Form Fieldset decorator.