Document Sets and Managed Metadata fields

I can successfully create a document set and add a Managed Metadata field to the FieldRefs of the document set's content type. All works fine. However, when I add the same MM fields to the WelcomPageFields I always get an error.

The error is

Object reference not set to an instance of an object. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  

Stack Trace: 


[NullReferenceException: Object reference not set to an instance of an object.]
   Microsoft.Office.DocumentManagement.DocumentSets.WelcomePageFieldCollection.FetchCollection() +477
   Microsoft.Office.DocumentManagement.DocumentSets.WelcomePageFieldCollection..ctor(SPContentType ctDocSet) +168
   Microsoft.Office.DocumentManagement.DocumentSets.DocumentSetTemplate.get_WelcomePageFields() +56

and the MM fields I am entering in the WelcomePageFields mark up is

<WelcomePageField ID="DE3C64DA-D6EB-4D3F-AA30-A047BFBB29B8" /><!-- MM field--><WelcomePageField ID="AF71FB25-F515-4FB9-A6EF-69737852A93C" /><!-- MM field note field-->

Any ideas please