Why is it throwing a "must be a string" error when using checkboxlist?

Question

Yii

I want to use checkboxlist in my yii2 site.
Trying to use it like this

<?= $form->field($Form, 'techies')->checkboxList(['no'=>'Нет', 'yes_camera'=>'Да,только камера', 'yes_both'=>'да,компьютер и камера'])->label(false) ?>

When you try to submit the form, it gives an error 'techies must be a string', as I understand it, that it does not pass validation. But here both the key and the value are text.
Rules:

[ 'techies', 'english_level', 'photo_1', 'photo_2', 'photo_3', 'photo_4', 'photo_5'], 'string'],

So what is the correct way to use this checkbox list?