What steps will reproduce the problem?
1. declare KeyProperty on Model

class Test(model.Model):
    k = model.KeyProperty()
    value = model.IntegerProperty()

2. filtering KeyProperty with None

Test.query(Test.k == None).count() # error
Test.query(Test.k != None).count() # error also

What is the expected output? What do you see instead?

File 
"/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-de
fault.bundle/Contents/Resources/google_appengine/google/appengine/ext/webapp/_we
bapp25.py", line 701, in __call__
    handler.get(*groups)
File "/Users/reiot/Documents/Works/ndbtest/main.py", line 91, in get
    count = Test.query(Test.k == None).count() # error
File "/Users/reiot/Documents/Works/ndbtest/ndb/model.py", line 500, in __eq__
    return self._comparison('=', value)
File "/Users/reiot/Documents/Works/ndbtest/ndb/model.py", line 491, in 
_comparison
    return FilterNode(self._name, op, self._datastore_type(value))
File "/Users/reiot/Documents/Works/ndbtest/ndb/model.py", line 1138, in 
_datastore_type
    return datastore_types.Key(value.urlsafe())
AttributeError: 'NoneType' object has no attribute 'urlsafe'

What version of the product are you using? On what operating system?

0.7 & 0.8

Please provide any additional information below.