Undefined in generating avro bytes.

avro_bug.hs

{-# LANGUAGE OverloadedStrings #-}
module Main where

import Data.Avro
import Data.Avro.Schema

data Thing = Thing { num :: Int }

thingSchema :: Schema
thingSchema = Record "Thing" Nothing [] Nothing Nothing
  [fld "num" Int]
  where fld nm ty = Field nm [] Nothing Nothing ty Nothing

instance HasAvroSchema Thing where
  schema = pure thingSchema

instance ToAvro Thing where
  toAvro p = record thingSchema
             ["num" .= num p]

dataset :: [Thing]
dataset = [Thing 5]

main :: IO ()
main = do
  bytes <- encodeContainer [dataset]
  print bytes

gistfile1.txt

When run, gives:
```
Prelude.undefined
CallStack (from HasCallStack):
  error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
  undefined, called at src/Data/Avro/Encode.hs:100:33 in avro-0.2.0.0-7D9gyFVmhinCwIHovPxGOA:Data.Avro.Encode
  ```