default_checkout_country hook not working in 3.0.5

Hi,

after updating my test site to 3.0.5, I get this error at checkout:

Notice: The “default_checkout_country” hook uses out of date data structures and の使用はバージョン 3.0.5 から非推奨になっています! 代わりに default_checkout_billing_country を使ってください。 in /home/amicamak/staging/2/wp-includes/functions.php on line 3830

Notice: Undefined index: AU in /home/amicamak/staging/2/wp-content/plugins/woocommerce/includes/class-wc-countries.php on line 217

The code I use to change the default checkout country on my site is:

add_filter( 'default_checkout_country', 'ap_ja_change_default_checkout_country' );

function ap_ja_change_default_checkout_country() {
  return 'JP'; // country code
}

It used to work until woocommerce 3.0.4

Thank you for your help
Alessio