Linq to SQL Query with null values

I am trying to convert a SQL query to a Linq query, but not getting the expected results.

SQL Query
SELECT     ST.stateid, ST.statename, SM.userid
FROM         dbo.tbl_servicearea_matrix AS SM RIGHT OUTER JOIN
                      dbo.tbl_states AS ST ON SM.stateid = ST.stateid
WHERE     (SM.userid = @userid) OR
                      (SM.userid IS NULL)

Linq Query
var rsServiceArea = from ST in DB.tbl_states
                                join SM in DB.ServiceAreas on ST.stateid equals SM.stateid                                
                                where SM.userid == IMOSAPI.GetActiveUserID || SM.userid == null
                                select new {ST.stateid,ST.statename,SM.userid};
        }

My SQL query returns all the states and the Linq only returns the states with userid.