GADT question

[J. Garrett Morris]

Hello,

I've been attempting to use GADTs to create a small domain specific
language, and I'm running into an odd problem.  Adding one case to the
example at the beginning of the Wobbly types paper:

data Term :: * -> *
    where Lit :: a -> Term a
          Inc :: Term Int -> Term Int
          IsZ :: Num a => Term a -> Term Bool
          Div :: Fractional a => Term a -> Term a -> Term a
          If :: Term Bool -> Term a -> Term a -> Term a

and extending the eval function accordingly:

eval :: Term a -> a
eval (Lit i) = i
eval (Inc t) = eval t + 1
eval (IsZ t) = eval t == 0
eval (Div t u) = eval t / eval u
eval (If cond cons alt) = if eval cond then eval cons else eval alt

I get an error "No instance for (Fractional a) arising from the use of
'/'"  This seems odd to me, since Div is constrained to have
fractional arguments.  Is there something obvious I'm missing?

Thanks,

 /g