Weird Purescript Error Mesaage...

Main.purs

class ParseBodyRL (rl :: RowList) b | rl -> b where
  parseBodyRL :: RLProxy rl  -> Request -> ExceptT String Aff b

instance parseBodyRLConsNil ::  ParseBodyRL RL.Nil Unit where 
  parseBodyRL _ = pure unit 
  
-- Weird error message...
--  Could not match type

--     Unit

--   with type

--     ExceptT String Aff Unit


-- while trying to match type t0 t1
--   with type Request -> ExceptT String Aff Unit
-- while checking that expression pure unit
--   has type Request -> ExceptT String Aff Unit
-- in value declaration parseBodyRLConsNil

-- where t0 is an unknown type
--       t1 is an unknown type