Email Invoice: 500 Error in AJAX response

With the new invoice functionality enabled, view a contribution and choose to Email Invoice. After submitting the form, there is a 500 error in the console and the form spins forever. The email is sent, and the invoice is correctly attached as a PDF file. So the issue must come after the form has processed and during the response or redirect step. Also interesting to note that if you open the URL from the console in a separate window, it doesn't error.

This is reproducible on the demo site.