Email location type must be 'primary' in profile or user creation won't work for event

I have created a free event with registration using a profile with "user account creation" required.

The location type of the email must be set to primary. Any other location such as "home" will result in an "error: email required" after form submission.

I am not sure if this is intentional or a bug, but it is not documented that I can see. Can someone point out the documentation or advise? Thanks.