Dynamic form elements to an displayField in Zend

I'm working on a application where users can be placed in a group. The groups can be added/edited in the database so these are dynamic. I've got the following form:

public function init()
{
    $this->setMethod('post')
         ->setAttrib('id', 'userGroup');

    //get the stuff out of the db
    $group = new Application_Model_GroupMapper();
    $disciplines = $group->fetchAll(array(
        'type' => 'discipline',
        'orderby' => 'g.name',
        'sort' => 'ASC'
    ));

    $disciplineFields = array();
    foreach($disciplines as $row):
        $el = $row->name;
        $this->addElement('checkbox', $el, array(
            'required'      => false,
            'label'         => $el,
            'class'         => 'inputCheckbox'
        ));
        $this->$el->setCheckedValue('true');
        $this->$el->setUnCheckedValue('false');

        array_push($disciplineFields,$el);
    endforeach;

    //discipline information
    $this->addDisplayGroup(
        $disciplineFields,
        'disciplineInformation',
        array('legend' => 'Discipline')
    );

BUT I get the error:

No valid elements specified for display group

Well that's strange because when i count my array $disciplineFields it has 4 items and the fields are echoed when I'm removing the displayGroup line. Also when I modify the displayGroup line to

//discipline information
$this->addDisplayGroup(
    array('Schipper'), //this is one of the records in the database
    'disciplineInformation',
    array('legend' => 'Discipline')
);

The 'Schipper' field is showed in the fieldset/displayGroup.

Why is this not working? What am I doing wrong?

You should put a breakpoint on Zend_Form::addDisplayGroup and debug it. I see nothing wrong in your code, perhaps your data are not correctly retrieved.

If you make a Zend_Debug::dump on disciplines, what is displayed ?

And you should pass your discipline via a setDisciplines method of your form instead of hard code it in the init.