ERROR: This username is invalid because it uses illegal characters.

Hi guys,

My site is an online entertainment outlet and I add new users from time to time and I’ve had no issues with this until today. I tried adding a new user as Ivan Cazares but I keep getting the following error message:

ERROR: This username is invalid because it uses illegal characters. Please enter a valid username.

I saw on some forums to try enter the name in lower case, which I did but I continue to get the error message. I don’t know what else to do. Any help would be greatly appreciated, thank you.

My site is IrishFilmCritic.com