"Set Password on Multisite Blog Creation" error on control panel user creation

Hi, I am getting an error when I try to create a new user in this page http://xxxxxx.com.br/wp-admin/user-new.php

in which the "set password" plugin doesn`t require a password, but cause this error:

Notice: Undefined index: key in /home/xxxx/public_html/xxxxx/wp-content/plugins/signup-password/signup-password.php on line 134 Warning: Cannot modify header information - headers already sent by (output started at /home/xxxx/public_html/xxxxxxx/wp-content/plugins/signup-password/signup-password.php:134) in /home/xxxx/public_html/xxxxxxx/wp-includes/pluggable.php on line 896

What should I do about that? Thank you.