Index: signup.module
===================================================================
RCS file: /cvs/drupal-contrib/contributions/modules/signup/signup.module,v
retrieving revision 1.129
diff -u -p -r1.129 signup.module
--- signup.module	26 Jan 2008 03:02:39 -0000	1.129
+++ signup.module	16 May 2008 17:08:04 -0000
@@ -1357,7 +1357,14 @@ function signup_sign_up_user($signup_for
 
     // Allow other modules to inject data into the user's signup data.
     $extra = module_invoke_all('signup_sign_up', $node);
-    $signup_form_data = serialize(array_merge($signup_form['signup_form_data'], $extra));
+    $signup_info = array();
+    if (!empty($signup_form['signup_form_data'])) {
+      $signup_info = $signup_form['signup_form_data'];
+    }
+    if (!empty($extra)) {
+      $signup_info = array_merge($signup_info, $extra);
+    }
+    $signup_form_data = serialize($signup_info);
 
     // Figure out if confirmation or reminder emails will be sent and
     // inform the user.