User Registration - Username is not valid error

Hi,

I’m building a simple user registration form - I only want the user to enter first name, last name and email address.

My understanding is that the email is used as the username if left blank.

However, I get a Username is not valid error error.

Same issue if I have a hidden field for the username.

This is confusing, the help text for the Field: User name field in the User Registration action:

“Auto-generated if form only requires email address for registration.”

Do I need to set this anywhere in WP? To specify that only email is required?

Any ideas?