[Haskell-beginners] error when unregistering a package

I always have trouble with conflicting packages when I do a fresh install of the Haskell Platform on an Ubuntu machine. Last time, I unregistered lots of packages, and let the necessary ones get recompiled. That fixed things. However, this time I'm having trouble with the unregister step. For many packages I get an error about a file not existing, e.g. $ sudo ghc-pkg unregister QuickCheck ghc-pkg: /var/lib/ghc-6.12.1/package.conf.d/QuickCheck-2.1.0.3-d4ae7d01dc2504e1785e18cd555e91ed.conf: removeLink: does not exist (No such file or directory) However, for many other packages, it works fine. Anyone know how to unregister a package when you get this error?