Form submit broken when switching between new and existing cards

The form will not submit when "New credit card" has been selected and then deselected.

User scenario / steps to reproduce: User fills out payment form, goes to review page. User then chooses to edit Payment Information, is taken back to previous page. If the user now selects New credit card, and then without filling out the form selects the previously entered card instead, the form can no longer be submitted. Attempts to click on the submit button will fail silently.

Cause:
The issue seems to be that square's JS is attempting to validate fields in iframes which no longer exist on the page.

JS console output:

Uncaught TypeError: Cannot read property 'postMessage' of null
    at n.blurAllInputs (paymentform:1)

and

Uncaught TypeError: Cannot read property 'postMessage' of null
    at n.setPostalCode (paymentform:1)
    at n.setPostalCode (paymentform:1)
    at HTMLButtonElement.requestCardNonce (eval at globalEval (jquery.min.js?v=2.2.4:2), <anonymous>:158:19)
    at HTMLButtonElement.dispatch (jquery.min.js?v=2.2.4:3)
    at HTMLButtonElement.r.handle (jquery.min.js?v=2.2.4:3)