"is_null" and  ">> None" operator  is parsed as "IS None" instead of  "IS NULL"
Since the value is parameterized, it will go in as the string 'NULL'.
 thank you very mach. you are right. issue was that i tried to run it with `django_model.objects.raw(sql_query,params)` and django orm putted `None` from `params` as `None` instead of `NULL`.

`column_name.is_null(True)` and `column_name >> None` expressions in `where` clause are returning `WHERE column_name IS None` in postgresql query.
i think just changing  [`db_value`](https://github.com/coleifer/peewee/blob/master/peewee.py#L928) to

``` python
  def db_value(self, value):
        """Convert the python value for storage in the database."""
        return 'NULL' if value is None else self.coerce(value)
```

will help
p.s i was using models translated from django
