error when exporting from civimember

Steps to replicate:

1. Add Custom Data of type Membership. (add any 2 fields)
2. Export Memberships (Select Fields)
3. Now delete 1 custom field that was created in step 1
4. Now repeat Step 2.

You will be get backtrace mentioned on forum : http://forum.civicrm.org/index.php/topic,4676.0.html