SilverStripe 4 beta3 TreeDropdownField给出错误：SourceObject必须是DataObject的子类

I must be missing something simple...I'm using the documentation here: http://api.silverstripe.org/en/master/class-SilverStripe.Forms.TreeDropdownField.html and I think I'm doing this correctly, but getting the error : SourceObject must be a DataObject subclass

This is my code

<?php
use SilverStripe\CMS\Model\SiteTree;
use SilverStripe\Forms\TreeDropdownField;
use SilverStripe\Forms\TextField;
use SilverStripe\Forms\TextareaField;
class HomePage extends Page {
    private static $db = array(
        'CTA'       => 'Varchar(255)',
        'LinkText'  => 'Varchar(50)'
    );
    private static $has_one = array(
        'PageLink'  => 'SiteTree'

    );
    public function getCMSFields() {
        $fields = parent::getCMSFields();
        $fields->addFieldToTab('Root.Main', TextareaField::create('CTA', 'Call To Action Text'), 'Content');
        $fields->addFieldToTab('Root.Main', TreeDropdownField::create('PageLinkID', 'What page does this call-to-action link to?', 'SiteTree'), 'Content');
        $fields->addFieldToTab('Root.Main', TextField::create('PageLinkText', 'Call To Action Link Text (Example: "Learn more")'), 'Content');
        $fields->removeByName('Content');
        return $fields;
    }
}