opencv createVideoWriter needs a more restrictive type?

main.hs

{-# LANGUAGE DataKinds #-}
module Main where

import OpenCV.HighCV
import OpenCV.Core.Image

main = do
    cap <- createFileCapture "in.avi"
    Just img <- cap
    w <- createVideoWriter "out.avi" ('M','J','P','G') 1 (width img, height img)
    w ( convertRGBToGray (img :: Image Trichromatic Word8 NoROI) )

-- fails (segfault, or fails to actually produce out.avi). Works just fine
-- when replacing the convertRGBToGray with id
--
-- so should createVideoWriter produce a function of type:
--
-- Image Trichromatic d r -> IO ()