Error 117: Invalid step operation



I'm working on a DTS connecting to Dynamics 365 online (Version 1612 ( using Dynamics 365 v5.5.5 adapter with SQL Server 2014 as a source.


I have an Update/Insert step that is performing very slowly, so I did some research and discovered Upsert may be a better choice.


I changed the Update/Insert step to Upsert and now that step, when testing, returns 'Error 117: Invalid step operation'. Also, when I run the DTS, that Upsert step creates duplicate records of existing entities, rather than the expected update existing or insert new.


I have a Lookup Link on an Account Number field in source, to the corresponding field in Target.


Unique index in Target entity is on the entity GUID which has no corresponding field in the source.


Is there something I'm missing here?