Can't derive Generic1 though

fieldnames.hs

class FieldNamesB b where
  bFieldNames :: IsString a => b (Const a)

  default bFieldNames :: (Generic1 b, FieldNamesB (Rep b), IsString a) => b (Const a)
  bFieldNames = to bFieldNames

instance FieldNamesB U1 where
  bFieldNames = U1

instance (FieldNamesB t) => FieldNamesB (M1 C m t) where
  bFieldNames = M1 bFieldNames

instance (FieldNamesB t) => FieldNamesB (M1 D m t) where
  bFieldNames = M1 bFieldNames

instance (FieldNamesB f, FieldNamesB g) => FieldNamesB (f :*: g) where
  bFieldNames = bFieldNames :*: bFieldNames

instance (m ~ 'MetaSel ('Just name) su ss ds, IsString a, KnownSymbol name) => FieldNamesB (M1 S m (Rec1 (Const a))) where
  bFieldNames = M1 $ Rec1 $ Const $ fromString $ symbolVal (Proxy :: Proxy name)