[Plugin: Sidebar Login] PHP Warning: call_user_func_array()

I’m not sure if this has always been happening on my server since installing this plugin or whether it happened after an upgrade. I’m still in the stages of building the site, but is there any way to correct the error below?

[warn] mod_fcgid: stderr: PHP Warning: call_user_func_array() [<a href='function.call-user-func-array'>function.call-user-func-array</a>]: First argument is expected to be a valid callback, 'wp_sidebarlogin_css was given in /home/mysite/public_html/wp-includes/plugin.php on line 395