Validation Form

I post data via AJAX on the server i get it like

[

name => ''
sex=> ''

]

My validation

'name' => [
            'label' => ' ',
            'rules' => 'required'
        ],

       'sex' => [
'label' => ' ',
'rules' => 'in_list[male,female]',
],

But when i load $this->validator->getErrors();

On"sex" field i got error "this field must be in list", but "sex" field is not required, why is happening?
I need next actions when field is filled check in list, when field is empty dont check

permit_empty