Subst in unbound broken

subst_broken.hs

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE MultiParamTypeClasses #-}
module Test where
import GHC.Generics
import Unbound.Generics.LocallyNameless


type Var = Name Factor
data Expr = SumOf [Summand]
          deriving (Show, Generic)
data Summand = ProductOf [Factor]
          deriving (Show, Generic)
instance Subst Var Expr
instance Subst Var Summand

data Factor = V Var
            | C Int
            | Subexpr Expr
          deriving (Show, Generic)

instance Subst Var Factor where
  isvar (V v) = Just (SubstName v)
  isvar _     = Nothing

[1 of 1] Compiling Test ( test.hs, test.o )

test.hs:27:23: error:
• Couldn't match type ‘Factor’ with ‘Name Factor’
arising from a use of ‘SubstName’
• In the first argument of ‘Just’, namely ‘(SubstName v)’
In the expression: Just (SubstName v)
In an equation for ‘isvar’: isvar (V v) = Just (SubstName v)
|
27 | isvar (V v) = Just (SubstName v)