F() expression not compatible with __range field look up

[Adonis Liu]

platform: Django-1.5, python2.7

code：

class TestModel(models.Model):

    a = models.SmallIntegerField()

    b = models.SmallIntegerField()

TestModel.objects.filter(a__range=(F('b')-1, F('b')+1)

result：

django.db.models.fields in get_prep_value

TypeError: int() argument must be a string or a number, not 'ExpressionNode'

Why：

Does anyone saw this error before, is it Django's bug?