max_value validation is inconsistant

[bobbo717@…]

I am attempting to use validation in the following modelform:

class BatchForm(ModelForm):

amount = forms.DecimalField(max_digits=8,decimal_places=2,max_value=599999.99)
class Meta:

model = Batch
exclude = ('submit_date','user')

Sometimes is_valid() returns true and sometimes it returns false with the amount 1212.

The error returnedis:

Ensure this value is less than or equal to 599999.99.

Is this a know issue?

Version Django-1.0.2-final Python 2.5.1 Mac OSX 10.5.7