See: http://groups.google.com/group/mongodb-user/browse_thread/thread/8a8131a6af1daf60

As a result of BsonDocument functional construction ignoring C# nulls (just like XDocument), the result of the following query builder expression can be surprising:

Query.EQ("x", null) => {}

This either needs to be documented more thoroughly, or if we choose to disallow this then Query.EQ should throw an ArgumentNullException.