How come I cannot update my signature?

Is this a feature for premium members only?

I see many people where very long, illustrious, multi-line signatures... when I try to update mine it complains about it being more than one line or more than 100 characters.

Am I missing something?

 

Yes, Premium membership includes, among other things, the ability to have multiple lines (4) and roughly twice more characters in the signature versus a standard account.