Querying for None in Datastore

I got a weird problem with GAE Datastore, when tried to search for None value. If I use gql, then the query works as expected:

from game.models import *
for c in Cell.gql("WHERE game=:g", g=None):
 print c

The above code prints the expected cells which are not bound to any game. But I need to iterate through cells of a certain board type, so instead of Cell.gql I start from board.cell_set and am trying to define a filter on game=None. The following code should give the same outcome as the previous one:

from game.models import *
for c in Cell.all().filter("game=", None):
 print c

But this time I get no results. Why?