Validation Messages in model

Why validationMessages in model cant reference to validation config file?

This is my model

PHP Code:

protected $validationRules      = 'formUser';
protected $validationMessages   = 'formUser'; 


So the validationRules work fine but the messages that return is not my custom message
so is this a bug or i do something wrong

This is my validation config file

PHP Code:

public $formUser = [
        'nama' => [
            'rules'  => 'required|min_length[5]',
            'errors' => [
                'required' => 'Nama tidak boleh kosong.',
                'min_length' => 'Nama user harus lebih dari 5 huruf',
            ]
        ],
        'username'    => [
            'rules'  => 'required|min_length[5]',
            'errors' => [
                'required' => 'Username tidak boleh kosong',
                'min_length' => 'Username harus lebih dari 5 huruf',
            ]
        ],
        'password'    => [
            'rules'  => 'required|min_length[5]',
            'errors' => [
                'required' => 'Password tidak boleh kosong',
                'min_length' => 'Passwrord harus lebih dari 5 huruf',
            ]
        ],
        'outlet'    => [
            'rules'  => 'required',
            'errors' => [
                'required' => 'Harap pilih outlet',
            ]
        ],
        'role'    => [
            'rules'  => 'required',
            'errors' => [
                'required' => 'Harap pilih peran untuk user',
            ]
        ],
    ];