Why isn't this hole's context normalizing?

Question

On the following program:

module Bytes

import Data.Fin

%default total

Byte : Type
Byte = Fin 256

fromNat : Nat -> Byte
fromNat n = maybe FZ (\x=>x) (natToFin n 256)

toNat : Byte -> Nat
toNat FZ     = Z
toNat (FS b) = S (toNat b)

prop : (b : Byte) -> fromNat (toNat b) = b
prop FZ     = ?prop_rhs_1
prop (FS x) = ?prop_rhs_2

The hole ?prop_rhs_1 is not normalizing correctly. Idris says its context is:

--------------------------------------
prop_rhs_1 : maybe (Delay FZ) (Delay (\x => x)) (natToFin _ 256) = FZ

Notice it is displaying (natToFin _ 256) instead of (natToFin Z 256), which should normalize to Just FZ.