Problem with AsString cast in BsonDocumentCollection

[object]

The following query works fine:

        var people = (from p in Collection.AsQueryable()
                      where p.FirstName.Contains("oe")
                      select p).ToList();

But this one always returns an empty result set:

        var people = (from p in BsonDocumentCollection.AsQueryable()
                      where p["fn"].AsString.Contains("oe")
                      select p).ToList();

It appears that if I cast a BsonValue using AsString, no results will be fetched.

It would be nice to have support for this, otherwise query on BsonDocumentCollection is quite limited.