Python: if more than one of three things is true, return false

I'm writing a django model that allows my site to have coupons.

Coupons can have three types: lifetime account voucher, certain period of months voucher, certain number of dollars voucher.

To keep things simple, I'm only allowing coupons to have one of the three possible values (i.e. a voucher can't be for $10 and 5 months). But I want to check when a coupon is being saved to ensure this rule is true.

Currently I have:

true_count = 0
if self.months:
    true_count += 1
if self.dollars:
    true_count += 1
if self.lifetime:
    true_count += 1    

if true_count > 1:
    raise ValueError("Coupon can be valid for only one of: months, lifetime, or dollars")  

I know there's a better way to do this, but I'm not seeing it (call it coder's block).

Help is much appreciated.

In case it maters, the three types are int, int, and bool

months = models.IntegerField(default=0)
cents = models.IntegerField(default=0)
#dollars = models.FloatField(default=0.00)
#dollars replaced with integer cents per advice of group
lifetime = models.BooleanField(default=False)