Null strings and LINQ to Entities

I've got quite a strange thing happening on an ASP.NET 4.0 web application using EF 4.0 as its database backend. Essentially, I've got a table that stores users' password reset requests (containing a reset key of type byte[], an expiry of type DateTime, and a foreign key to a User containing a string Email and string Name). Some users do not have an email address set, so for a PasswordRequest request, request.Email is null.

Here's the problem. This works perfectly fine:

string u = Request["u"];
string e = Request["e"];

var requests = from r in context.PasswordRequests
               where r.User.Name == u && r.User.Email == null && r.Expiry >= DateTime.Now
               select r;

I get the expected number of results (nonzero, since there are entries with null emails).

But this always returns an empty collection when e is null:

string u = Request["u"];
string e = Request["e"];

var requests = from r in context.PasswordRequests
               where r.User.Name == u && r.User.Email == e && r.Expiry >= DateTime.Now
               select r;

The only thing that I got to work properly (which doesn't logically make any sense) is this:

string u = Request["u"];
string e = Request["e"];

IQueryable<PasswordRequest> requests;

if (e == null)
    requests = from r in context.PasswordRequests
               where r.User.Name == u && r.User.Email == null && r.Expiry >= DateTime.Now
               select r;
else
    requests = from r in context.PasswordRequests
               where r.User.Name == u && r.User.Email == e && r.Expiry >= DateTime.Now
               select r;

I'm absolutely stumped. Any ideas?