Unable to validate UBL instance

LS,

I am trying to validate an instance of an UBL invoice using a receive pipeline with an XMLValidator component but receice the following error:

"The content type of the base type must not be a simple type definition.".

I am able to validate the same instance within Visual Studio using the schema.

Is this a bug in BizTalk, as I find similar issues on the net regarding this error, but for different situations.
http://social.msdn.microsoft.com/Forums/en/biztalkgeneral/thread/9fd4e5a1-d19f-4df7-b8c7-b0fba8b87b81
http://support.microsoft.com/kb/972752

Regards,

René