Elm bug: `Cannot read property 'tag' of undefined`

ElmBug.elm

module ElmBug exposing (..)

import Json.Decode as JD
import Json.Helpers as JH

import Dict

sumDecoder : String -> List (String, a) -> List (String, JD.Decoder a) -> JD.Decoder a
sumDecoder name nullaryList singleFieldList = JD.oneOf
  [ JH.decodeSumUnaries name
      (Dict.fromList nullaryList)
  , JH.decodeSumObjectWithSingleField name
      (Dict.fromList singleFieldList)
  ]
type Condition
  = RecurringEffect Effect
  | Dead
  | Incapacitated
  | AddDamageBuff Int

type Effect
  = ApplyCondition ConditionDuration Condition
  | Heal Int
  | Damage Int
  | MultiEffect (List Effect)
  | GenerateEnergy Int

type ConditionDuration
  = Interminate
  | Duration Int

conditionDecoder = sumDecoder "Condition"
  [ ("Dead", Dead)
  , ("Incapacitated", Incapacitated)]
  [ ("RecurringEffect", JD.map RecurringEffect lazyEffectDecoder)
  , ("AddDamageBuff", JD.map AddDamageBuff JD.int)]

lazyEffectDecoder = JD.lazy (\_ -> effectDecoder)

effectDecoder =
  JH.decodeSumObjectWithSingleField "Effect"
    (Dict.fromList
      [ ("Heal", JD.map Heal JD.int)
      , ("Damage", JD.map Damage JD.int)
      , ("GenerateEnergy", JD.map GenerateEnergy JD.int)
      , ("ApplyCondition", JD.map2 ApplyCondition
                                     (JD.index 0 conditionDurationDecoder)
                                     (JD.index 1 conditionDecoder))
      , ("MultiEffect", JD.map MultiEffect (JD.list lazyEffectDecoder))
      ])

conditionDurationDecoder = sumDecoder "ConditionDuration"
  [("Interminate", Interminate)]
  [("ConditionDuration", JD.map Duration JD.int)]


-- > JD.decodeString ElmBug.conditionDecoder "{\"RecurringEffect\": {\"GenerateEnergy\": 10}}"
-- TypeError: Cannot read property 'tag' of undefined