Notice: Undefined index: form in /home/www/cliniciancorps.org/wp2/wp-content/plu

Why am I getting this error with this plug-in?

Notice: Undefined index:

form in /home/www/cliniciancorps.org/wp2/wp-content/plugins/simplesecure/simplesecure.php on line 155

Here is the code:

$output .= '<input name="ss_subject" type="hidden" value="Form ' . ($params['form'] == 'B' ? 'B' : 'A') . '" />';