session id null in sys.dm_os_tasks We are seeing parallelism causing worker thread shortage in SQL,while investigating we found session id is null for some sessions,Below is the query used.What does this session id null means ?

select session_id,count(*) as 'totalthreads' from sys.dm_os_tasks
group by session_id
having count(*)&gt;1
order by totalthreads desc


Output varies as below:
The session is sleeping if the session_id value of sys.dm_os_tasks is NULL. This is stated in the link above under the Examples section: Associating Session IDs with Windows threads.


  The following query does not return information for sessions that are
  sleeping.