Rest framework serializer always returns False for is_valid

My serializer, quite basic:

class TestSerializer(serializers.Serializer):
    date_time = serializers.DateTimeField()

wanted to try out from shell but I get False each time I try to check it for validation.

> import datetime
> s=TestSerializer({'date_time': datetime.datetime(year=2012,month=12,day=12)}
> s.data
{'date_time': datetime.datetime(2012, 12, 12, 0, 0)}
> s.is_valid()
False
> s.errors
{u'non_field_errors': [u'No input provided']}

Why is this? What is going on?