Index: civievent_discount.module
===================================================================
--- civievent_discount.admin.inc	(revision 374)
+++ civievent_discount.admin.inc	(working copy)
@@ -333,7 +327,7 @@
     $track = array('id' => $pid, 'type' => 'Event', 'description' => $params['description']);
 
   // Membership
-  } else if (in_array($class, array('CRM_Contribute_Form_Contribution_Confirm'))) {
+  } else if (in_array($class, array('CRM_Contribute_Form_Contribution_Confirm')) && !empty($params['membershipID'])) {
     $mid = $params['membershipID'];
     // Need to lookup the contact id if it's a 100% discount.
     if (!empty($params['contactID'])) { $contactid = $params['contactID']; }