Error when merging duplicate contacts.

After upgrading from CiviCRM version 4.5 to 4.6, we began getting the following error when trying to merge duplicate contacts:

" mandatory key(s) missing from params array: one of(activity_name, activity_type_id, activity_label) "

Fortunately, the duplicate contacts were merged successfully despite the error.

Is there a way to resolve this error? Has anyone else had this issue?