Problem when adding promoted properties to a schema - sometimes results in error reported in HAT - unable to find XPath to node

Microsoft.XLANGs.RuntimeTypes.XPathUpdateException: A failure occurred while evaluating the distinguished field Email against the message part data. The message part data does not contain at least one of the nodes specified by the XPath expression (listed below) that corresponds to the distinguished field. The cause for this error may be that the message part data has not been initialized or that the message part data does not conform to the message part schema. Ensure that the message part data is initialized correctly.

XPath expression: /*[local-name()='Client' and namespace-uri()='http://Customer.Target']/*[local-name()='Email' and namespace-uri()=']

   at Microsoft.XLANGs.Core.XSDPart.SetDistinguishedField(String dottedPath, Object val)

   at Customer.Orchestration_1.segment1(StopConditions stopOn)

   at Microsoft.XLANGs.Core.SegmentScheduler.RunASegment(Segment s, StopConditions stopCond, Exception& exp)
Microsoft.XLANGs.RuntimeTypes.XPathUpdateException

Reloading and saving the map file seems to solve this problem?