How to make fields required in CMS

I try to make fields required in the CMS:

class Documents extends DataObject {

    private static $db = array(
        'DocType' => 'Text',
        'DocTitle' => 'Text',
        'DocNumber' => 'Text'
    );

    public function getCMSFields() {
        $fields = parent::getCMSFields();   
        $fields = FieldList::create(TabSet::create('Root'));
        $fields->addFieldsToTab('Root.Main', array(
            DropdownField::create('DocType','Document Type'),             
            DropdownField::create('DocStatus','Document Status'),
            TextField::create('DocNumber','Document Number'),
            ...

            RequiredFields::create(array('DocType','DocTitle','DocNumber'));
        ));

        return $fields;
    }

But in my SilverStripe error log I get the following:

"Uncaught Exeption: the method 'getname' doesn't exist on RequiredFields or the method is not public".

How do I make fields required in the SilverStripe CMS?