Enfold and WooCommerce – Checkout issue with credit card

I’m using the Enfold theme with the WooCommerce plugin – I’ve noticed an issue on my checkout page. When someone enters a credit card number that I don’t support – e.g. American Express – I don’t get any visual feedback. I cannot click the Confirm and Pay button and the credit card number field isn’t highlighted in red like it is when you choose the default WordPress theme – see link to screenshot below for another site with the twenty fifteen theme.

Is there some kind of conflict here?