"Changes register item process on failed / failed with reason Value cannot be null." error after

UNIFYBroker/Plus importing AD Users into a locker.  The 'member' field is locker-to-adapter mapped.  When I change the member field value in AD and run an Import All on the AD users connector, the following error is logged:

20210118,04:24:11,UNIFYBroker,Change detection engine,Error,"Changes register item processing on failed.
Parameter name: collection. Duration: 00:00:00.0139980
Error details:
Parameter name: collection. Duration: 00:00:00.0139980
Error details:
System.ArgumentNullException: Value cannot be null.
Parameter name: collection
at System.Collections.Generic.HashSet`1..ctor(IEnumerable`1 collection, IEqualityComparer`1 comparer)
at Unify.Product.IdentityBroker.MultiRelationalTransformationContribution.GetChangedMultiValues(IEntityPair entityPair, Boolean relevantFieldsChanged)
at System.Linq.Enumerable.d__23`3.MoveNext()
at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
at Unify.Product.IdentityBroker.EntityPartitionPostgreSqlContextBase`3.GetEntitiesByFieldValues(TEntityKey field, IEnumerable`1 values)
at Unify.Product.IdentityBroker.MultiRelationalTransformationContribution.d__25.MoveNext()
at System.Linq.Enumerable.d__17`2.MoveNext()
at System.Linq.Enumerable.d__17`2.MoveNext()
at System.Linq.Enumerable.d__64`1.MoveNext()
at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
at Unify.Product.IdentityBroker.ChainedTransformationChangeProcessor.PublishChange(IEntityPair[] changedEntityPairs, DateTime changeProcessTime, ICollection`1 changeRecords)
at Unify.Product.IdentityBroker.ChainedTransformationChangeProcessor.ProcessChangeReport(IDictionaryTwoPassDifferenceReport`4 changesReport, DateTime changeProcessTime)
at Unify.Framework.Visitor.Visit[T](IEnumerable`1 visitCollection, Action`2 visitor)
at Unify.Product.IdentityBroker.ChangeReportProcessor.ProcessCurrentReport(IEnumerable`1 adapterTransformationProcessors, IDictionaryTwoPassDifferenceReport`4 differenceReport, DateTime changeTime)
at Unify.Product.IdentityBroker.ChangeReportProcessor.CreateAndProcessReport[T](ITransformationChangeProcessor[] adapterTransformationProcessors, ICollection`1 sourceEnumerable, DateTime changeTime, HashSet`1 invalidEntities, Action`2 addAction, Func`3 addCheck)
at Unify.Product.IdentityBroker.ChangeReportProcessor.ProcessReport(IChangeReportProcessingRequest request)",Normal

This is not urgent, just noting it here for completeness.  I don't believe it will impact the solution I'm currently working on.

See also: https://voice.unifysolutions.net/en/communities/6/topics/57-import-all-entitiesfrom-connector-workday-employee-failed-with-reason-value-cannot-be-null for the same error message - ticket closed but it may be that the underlying issue was not identified or fixed.