Biztalk 2013 <string xmlns='http://schemas.microsoft.com/2003/10/Serialization/'> was not expected - wcf

Question

I have a BizTalk 2013 orchestration which I published as a WCF service. The orchestration has a Request-Response port accepting a string and returning an entity which I have defined in a schema.
When I try use the service and pass a string to it, I get the error
http://schemas.microsoft.com/2003/10/Serialization/'> was not expected.
Does anyone have any ideas how to fix this?
Thanks
Full error details below:
xlang/s engine event log entry: Uncaught exception (see the 'inner
exception' below) has suspended an instance of service
'CAA.POC.ESB.BizTalk.Orchestrations.RetrieveAllCases(c408d4a7-3795-f6c7-dc95-c02edfe00740)'.
The service instance will remain suspended until administratively
resumed or terminated. If resumed the instance will continue from its
last persisted state and may re-throw the same unexpected exception.
InstanceId: 6b9cbf6a-2a63-4401-b3a1-05a870da963c Shape name:
ConstructCases ShapeId: 189c5a7f-2306-49dc-bd07-8430e638ba45 Exception
thrown from: segment 1, progress 7 Inner exception: There is an error
in XML document (1, 2).
Exception type: InvalidOperationException Source: System.Xml Target Site: System.Object Deserialize(System.Xml.XmlReader,
System.String, System.Xml.Serialization.XmlDeserializationEvents) The
following is a stack trace that identifies the location where the
exception occured
at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader
xmlReader, String encodingStyle, XmlDeserializationEvents events)
at System.Xml.Serialization.XmlSerializer.Deserialize(TextReader
textReader) at Microsoft.XLANGs.Core.Value.GetObject(Type t) at
Microsoft.XLANGs.Core.Value.RetrieveAs(Type t) at
Microsoft.XLANGs.Core.Part.ProtectedRetrieveAs(Type t) at
Microsoft.XLANGs.Core.Part.RetrieveAs(Type t) at
CAA.POC.ESB.BizTalk.Orchestrations.System_String.get_TypedValue()
at
CAA.POC.ESB.BizTalk.Orchestrations.RetrieveAllCases.segment1(StopConditions
stopOn) at
Microsoft.XLANGs.Core.SegmentScheduler.RunASegment(Segment s,
StopConditions stopCond, Exception& exp) Additional error information:
<string xmlns='http://schemas.microsoft.com/2003/10/Serialization/'> was not
expected.
Exception type: InvalidOperationException Source: System.Xml Target Site: System.Object Read_string() The following is a stack
trace that identifies the location where the exception occured
at
System.Xml.Serialization.XmlSerializationPrimitiveReader.Read_string()
at
System.Xml.Serialization.XmlSerializer.DeserializePrimitive(XmlReader
xmlReader, XmlDeserializationEvents events) at
System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader
xmlReader, String encodingStyle, XmlDeserializationEvents events)