Error after registration

After new user has registered, it shows error:
Fatal error: Undefined class constant ‘user_meta’ in /../../public_html/test.com/wp-content/plugins/user-activation-email/user-activation-email.php on line 352

When you check afterwards the user list, this new user is added, but breaking the site with this error is really bad and it needs to be fixed ASAP. I really need this to authenticate new users and this inconvenience is exactly what i didn’t need now..
I hope you take care of it as soon as possible because it’s a good plugin after all
thanks

http://wordpress.org/plugins/user-activation-email/