Orphaned Transaction using SQL Server 2005 and BizTalk 2006

On our BizTalk 2006 database server (SQL Server 2005 SP1), we have noticed that we have been getting a few orphaned transactions. Most of them can be explained or handled, with one exception. When we run the following query to determine the Orphaned transaction:

selectdistinct req_transactionuow, req_transactionIDfrom syslockinfowhere req_spid=-2

we see the follwing:

When we try to kill this UOW using the guid of all zeroes, we get the following error:

Msg 6110,Level 16,State 1, Line 1

Thedistributedtransactionwith UOW{00000000-0000-0000-0000-000000000000} doesnot exist.

Does anyone know why we might get a UOW of all zeroes, and why, when we try to kill it, it says it does not exist?For the record, if we run the first query again after the failed attempt to kill it, we see that UOW still exists.