GHC API panic (evaluated the place holder for a PostTcType)

The same code that works for GHC 7.10.2 blows up on GHC 7.8.4, with errors like

    test/ArgonSpec.hs:104: (best-effort)
      6) Argon.analyze accounts for && operator
           uncaught exception: GhcException (argon-test: panic! (the 'impossible' happened)
             (GHC version 7.8.4 for x86_64-unknown-linux):
           	fixity
           
           Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
           )
    
      test/ArgonSpec.hs:106: (best-effort)
      7) Argon.analyze counts everything in a real example
           uncaught exception: GhcException (argon-test: panic! (the 'impossible' happened)
             (GHC version 7.8.4 for x86_64-unknown-linux):
           	Evaluated the place holder for a PostTcType
           
           Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
           )

The responsible module is https://github.com/rubik/argon/blob/ghc-7.8/src/Argon/Visitor.hs and in particular (I think):

visitExp :: Exp -> Int
visitExp (GHC.HsIf {}) = 1
visitExp (GHC.HsMultiIf _ alts) = length alts - 1
visitExp (GHC.HsLamCase _ alts) = length (GHC.mg_alts alts) - 1
visitExp (GHC.HsCase _ alts)    = length (GHC.mg_alts alts) - 1
visitExp _ = 0

visitOp :: Exp -> Int
visitOp (GHC.OpApp _ (GHC.L _ (GHC.HsVar op)) _ _) =
    case getName op of
      "||" -> 1
      "&&" -> 1
      _    -> 0
visitOp _ = 0

somehow, even if I use wildcards, the first arguments of the type constructors gets evaluated and since it's undefined GHC crashes. What am I supposed to do? The documentation does not say anything on the matter.