NULL ::字符变化CakePHP 3表单

I get NULL::character varying as the value of a Form in CakePHP 3 when I expect it to be blank as I just created new entity. The controller code I use is

public function add() {
    $audience = $this->Audiences->newEntity();
    $this->set(compact('audience', 'languages'));
    $this->set('_serialize', ['audience']);

In the add.ctp my code to redner the form is

echo $this->Form->create($audience, ['novalidate' => true]);
echo $this->Form->input('audience', ['type' => 'text']);