No able to logged in with a Website User


I have a user type “Website User”, and the user doesn’t have any role assigned to him. I am not able to login with this user and receives an error message as shown below:

These fields are custom fields and I made it mandatory as per the business requirement,
Now, when I post this issue to support team they gave the reply as shown below:

Website users are added as Lead when they signup. Since you have these fields as mandatory, system is not able to create Lead for this user. Please make these fields non-mandatory, so that Lead can be created for this website user.

My problem is I cannot make those fields non-mandatory, what is the work-around of this issue?