C# linq: join with or condition

Question

I am trying to join 2 tables with an or condition. Here is what I have tried so far:

var dos = from d in dc.Dos
            join c in dc.Clis on (d.clid = c.id or d.clid is null)
            select new { id = d.id, clid = d.clid, cliname = c.Name }

But I receive syntax errors. How can I fix it? What I want is to receive the rows with d.clid = c.id and the rows with d.clid = null