Update External List Workflow Action Fails in SP 2013

Howdy Y'all.

We have a requirement to update an external list using WF.

We are using an ECT with a SSID targeting a SQL login. The ECT can be updated from the external list, all methods no problems.

For workflow actions (2010 only for updating external lists) all CRUD actions work except for "Update List Item"

The sql profiler shows that the correct account gets into sql and performs a query to find a match.

Here is the relevent SP log error entry:

High     Leaving Monitored Scope (SPListDataSource.FindMatchingEntity: EntityNamespace 'https://appsit/ Entity 'Test SS WF UPdate' Field 'ID' Value '7'.).

Medium   System.NullReferenceException: Object reference not set to an instance of an object.     at Microsoft.SharePoint.SPListDataSource.UpdateEntityInstance(String bdcid, SPListItem itemToUpdate, IDictionary dictFieldValues)     at Microsoft.SharePoint.Workflow.SPWinOEWSSService.CommitUpdateListItem(Transaction txn, Object[] transData)

Is this a bug?

This has been reproduced for different database servers, with and without a Secure Store ID on 2 different farms.

Thanks for the help and may you continue to grow and love,

Rudi