TH bug?

If tried to compile the Print.hs file below (based on the yesterday's mailing list discussion around the th printf example). I get the following error message (5.05, MAIN line, Nov 26th build), but I'm not sure if I did something really silly or if there's a genuine bug in the compiler.  Can you pls have quick look at the Print.hs source below and see if you can spot the reason for the ghc crash?

Thanks,
- Reto

bash-2.05a$ lghc -fglasgow-exts -package haskell-src  -i./src:./ext/HUnit-1.0 -odir./bin -hidir./bin -tmpdir./tmp -Wall -c ./src/Print.hs 
ghc-5.05: panic! (the `impossible' happened, GHC version 5.05):
rename/RnExpr.lhs:164: Non-exhaustive patterns in function rnExpr


Please report it as a compiler bug to glasgow-haskell-bugs@xxxxxxxxxxx,
or http://sourceforge.net/projects/ghc/.



module Print(pr) where

import Language.Haskell.THSyntax

data Format = D | S | L String

parse :: String -> [Format]
parse s   = [ L s ]

gen :: [Format] -> Expr
gen [D]   = [| \n -> show n |]
gen [S]   = [| \s -> s |]
gen [L s] = string s

pr :: String -> Expr
pr s      = gen (parse s)