commerce_payment_validate_credit_card_exp_date($month, $year) returns TRUE when $year starts with an alphabetic character

I just noticed that commerce_payment_validate_credit_card_exp_date('1', 'blah') will return TRUE.

The other functions inside of commerce_payment_credit_card_validate() handle numbers initially like so:

<?php
  // Ensure every character in the number is numeric.
  if (!ctype_digit($number)) {
    return FALSE;
  }
?>