Custom validation message not firing

I am sure it is something simple, but my matches[] validation message is showing the default message instead of my custom one. 

In my controller:

PHP Code:

$validation->withRequest($this->request)->run($_POST, 'update_password') 


In config/validation

   

PHP Code:

public $update_password = [
    'password'          => 'required|min_length[10]',
    'password_again'    => 'matches[password]'
];

public $update_password_errors = [
    'password' => [
        'required'    => 'Passwords must be at least 10 characters',
        'min_length'  => 'Passwords must be at least 10 characters'
    ],
    'password_again' => [
        'matches[password]' => 'Your passwords do not match'
    ]
]; 


The error I get when the passwords don't match is:

"The password_again field does not match the password field."

instead of "Your passwords do not match"

Did I typo something?

My form fields are password and password_again.
 

Rename the key matches[password] to matches