fail to raise PDOException

Env: Cake 3.1.4, PHP 5.6.14

//template
<?= $this->Form->postLink('Delete', ['action' => 'delete', $user->id], ['confirm' => __('Are you sure you want to proceed?')]) ?>

//controller
public function delete($id = null)
{
   $this->request->allowMethod(['post', 'delete']);
   $user = $this->Users->get($id);
   try {
      $this->Users->delete($user);
   } catch (Exception $e) {
       debug($e);
}

Result: PDOException is performed!

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