MailConverter: Update Ticket Issue

My customer is trying to use the MailConverter to add incoming emails to existing Tickets using the "Update Ticket" action and the "Has Ticket Number" match.

But when the process is trying to create the Comment to add to the Ticket it fails because there is no "assigned_to" value for the MailScannerRule object.

When you choose the "Update Ticket" action, the Assigned To field disappears so I would assume that the MailConverter would use either admin, or the owner of the Ticket... But I can't see anywhere where the MailScannerRule->assigned_to value is updated after initialisation...

Tue Apr 24 10:16:47 2018,757 [26529] DEBUG VT - Prepared sql query being executed : SELECT contactid FROM vtiger_contactdetails INNER JOIN vtiger_crmentity ON crmid = contactid WHERE
 setype = ? AND email = ? AND deleted = ?
Tue Apr 24 10:16:47 2018,758 [26529] DEBUG VT - Prepared sql query parameters : [Contacts,[email protected],0]
Tue Apr 24 10:16:47 2018,758 [26529] DEBUG VT - Caching Contact Id found for email: [email protected]
Tue Apr 24 10:16:47 2018,758 [26529] DEBUG VT - Caching Ticket [Test Ticket for Mail Converter]
Tue Apr 24 10:16:47 2018,758 [26529] DEBUG VT - Reusing Cached Contact Id for email: [email protected]
Tue Apr 24 10:16:47 2018,758 [26529] DEBUG VT - Entering getColumnFields(ModComments) method ...
Tue Apr 24 10:16:47 2018,758 [26529] DEBUG VT - in getColumnFields ModComments
Tue Apr 24 10:16:47 2018,758 [26529] DEBUG VT - Prepared sql query being executed : SELECT tabid, fieldname, fieldid, fieldlabel, columnname, tablename, uitype, typeofdata, presence
                FROM vtiger_field WHERE tabid in (?)
Tue Apr 24 10:16:47 2018,758 [26529] DEBUG VT - Prepared sql query parameters : [47]
Tue Apr 24 10:16:47 2018,759 [26529] DEBUG VT - Exiting getColumnFields method ...
Tue Apr 24 10:16:47 2018,759 [26529] INFO VT - PearDatabase ->TRANS Started
Tue Apr 24 10:16:47 2018,760 [26529] DEBUG VT - Prepared sql query being executed : insert into vtiger_crmentity (crmid,smcreatorid,smownerid,smgroupid,setype,description,modifiedby,
createdtime,modifiedtime,source) values(?,?,?,?,?,?,?,?,?,?)
Tue Apr 24 10:16:47 2018,760 [26529] DEBUG VT - Prepared sql query parameters : [16957,0,,0,ModComments,,0,2018-04-24 10:16:47,2018-04-24 10:16:47,CRM]
Tue Apr 24 10:16:47 2018,760 [26529] INFO VT - PearDatabase ->ADODB error  Query Failed:insert into vtiger_crmentity (crmid,smcreatorid,smownerid,smgroupid,setype,description,modifie
dby,createdtime,modifiedtime,source) values(?,?,?,?,?,?,?,?,?,?)::->[1048]Column 'smownerid' cannot be null
Tue Apr 24 10:16:47 2018,760 [26529] INFO VT - function insertIntoEntityTable ModComments vtiger_table name vtiger_modcomments