ghc: panic! (the 'impossible' happened)

ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.2 for x86_64-unknown-linux):
	initTc: unsolved constraints
  WC {wc_insol =
        [W] ask_an9K :: t_an9J[tau:1] (CHoleCan: ask)
        [W] put_an9Q :: t_an9P[tau:1] (CHoleCan: put)}

So this code derives an instance of the Decimal type incorrectly, but not sure if this is causing the error.

I have some nested records, which contain a few Decimals. I am trying to make them acidic.

instance SafeCopy (DecimalRaw a) where
  putCopy (Decimal d) = contain $ safePut d
  getCopy = contain $ Decimal <$> safeGet

deriveSafeCopy 0 'base ''Client
deriveSafeCopy 0 'base ''Article


deriveSafeCopy 0 'base ''ServerData
makeLenses ''ServerData

writeState :: String -> Update ServerData ()
writeState newValue
  = put (ServerData newValue)
queryState :: Update ServerData String
queryState = do ServerData string <- ask
                return string

makeAcidic ''ServerData ['writeState 'queryState]

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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