CRM 2013 Online: Update a lookup field to blank using workflow

Need to update a lookup field on the record to [blank] using workflow. Steps provided on below mentioned link doesn't seem to work on CRM 2013 online environment:

http://social.microsoft.com/Forums/en-US/806f2130-84ec-414c-ab51-d871d1d6a095/update-a-field-with-blank-value-in-a-workflow-crm-2011?forum=crmdevelopment

The reason is:

The Operator field is disabled in the 'Form Assistant' pane.

Early response is much appreciated.

Thanks,
Abhi