For those who are facing this issue with PO/PR workflow.
This problem arises because Event parameter 'Release Code' is not passed while triggering the event.
If you are using custom w/f template or copy of standard w/f template,it can be resolved by unchecking the mandatory check box in releasecode container in w/f template.
For other workflows(non PO PR) also,it can be improvised similarly.
Thanks
Manas
