Why doesn't using a nullable Guid work in this linq query?

Question

I have this code to find the root node of a tree:

Guid? currentNode = null; 
var root = db.RecursiveTrees.Where(x => x.ParentId == currentNode).ToList();

This query returns 0 results.

If I run this query I get the expected row returned:

var root = db.RecursiveTrees.Where(x => x.ParentId == null).ToList();

Why doesn't the first query work (using the latest version of entity framework)?

EDIT:

Workaround:

List<RecursiveTree> root;
if (nodeid == null)
   root = db.RecursiveTrees.Where(x => x.ParentId == null).ToList();
else
   root = db.RecursiveTrees.Where(x => x.ParentId == new Guid(nodeid)).ToList();     

Answer 1

This is a known bug in LINQ to Entities when dealing with nullable value-types. According to the relevant Connect issue, this will be fixed in the next release.