Build failure of Haskell example

HaskellServer.hs:73:60:
    Couldn't match type GHC.Int.Int32'
    Expected type: Maybe GHC.Int.Int32
      Actual type: Maybe Int
    In the ($)', namely
      Data.Text.Lazy.Internal.Text'
                with actual type Just', namely f_InvalidOperation_why' field of a record
    In the second argument of InvalidOperation
         {f_InvalidOperation_what = Just $ fromEnum $ op work,
          f_InvalidOperation_why = Just "Cannot divide by 0"}'