GeoDjango，两点之间的距离

Question

我有以下型号：

class City(models.Model):
    name = models.CharField(null=False, blank=False, max_length=200)
    coordinates = models.PointField(null=False, blank=False)
    objects = models.GeoManager()

我正在尝试使用以下内容找到上述两个城市之间的距离：

city1=City.objects.get(name='New York')
city2=City.objects.get(name='Boston')
city1.coordinates.distance(city2)

但是得到以下内容：

Traceback (most recent call last):
   File "<console>", line 1, in <module>
   File "/home/rok/apps/django-trunk/django/contrib/gis/geos/geometry.py", line 667, in distance
    raise TypeError('distance() works only on other GEOS Geometries.')
TypeError: distance() works only on other GEOS Geometries.

看结构和数据，似乎没问题：

>>> city1.coordinates.coords
(40.714623,-74.006605)
>>> city2.coordinates.coords
(42.360024, -71.060168)

我做错了什么？