Panic "tcTyVarDetails" in GHCi when :forcing a data family instance

I loaded the following code into GHCi:

{-# LANGUAGE GADTs, DataKinds, KindSignatures, TypeFamilies, PolyKinds #-}

data Nat = Zero | Succ Nat
data family Sing (a :: k)
data instance Sing (a :: Nat) where
  SZero :: Sing Zero
  SSucc :: Sing n -> Sing (Succ n)

Then. I run these commands at the prompt:

*Main> let x = SSucc SZero
*Main> :f x

The response is

ghc: panic! (the 'impossible' happened)
  (GHC version 7.7.20121031 for x86_64-apple-darwin):
	tcTyVarDetails
<<details unavailable>>

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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