The part '%messagepart%' of message '%multipart message%' contained a null value at the end of the construct block.

BizTalk throws the following error when one tries to create a multipart message with a message part that was just created from a map in the same construct shape:

Shape name: Send_1
ShapeId: 11910fdc-f4a1-4391-b09d-6a9bd0275ff8
Exception thrown from: segment 1, progress 11
Inner exception: The part '%messagepart%' of message '%multipart message%' contained a null value at the end of the construct block.

Exception type: NullPartException
Source: Microsoft.XLANGs.Engine
Target Site: Void CheckNonNull()
The following is a stack trace that identifies the location where the exception occured

at Microsoft.XLANGs.Core.Part.CheckNonNull()

Somehow, BizTalk looses the value of the message part that was NOT mapped.

A work-around for this is to create a second construct shape. One just creates the message part in a mapping, and the second shape puts the message parts together.

Thanks, Christian!