Impossible happened when using ViewPattern in Arrows

When I try to load file in ghci impossible happened:

ghc: panic! (the 'impossible' happened)
  (GHC version 6.12.1 for x86_64-unknown-linux):
	collectl/go
    ((=={v aOp} [lid]
      u{v avw} [lid]) -> {229:56-59}{229:56-59}ghc-prim:GHC.Bool.True{(w) d 6u})

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Problematic function:

isOn :: ArrowChoice a => DSignal a c -> SF a b (Maybe c)                        
isOn (DSignal _ f u) = let sf = SF $! \_ -> proc (_, s, e) ->                   
                             case fromDynamic (_payload e) of                   
                                 Just (Event ((== u) -> True) v) ->             
                                     returnA -< (Just (f v), s, sf)             
                                 Nothing -> returnA -< (Nothing, s, sf)         
                       in sf

(Full sources will be attached. As it was found in-development so style may not be perfect)