Inviting existing user's email address gives error "Please enter a valid email address. " (BB-6049)

The email address is valid. I have tried this with more than one address. They do not contain any special characters; in fact when trying the same address but with the google trick of +something, it works. For example user@gmail.com is invalid (already registered actually), whereas user+bitbucket@gmail.com is valid.

The error message is incorrect.