Can't install packages with my inplace compiler

When I say cabal install --with-ghc=/home/simonpj/5builds/HEAD-2/inplace/bin/ghc-stage2 I get this ghc-stage2: ghc no longer supports single-file style package databases (dist/package.conf.inplace) use 'ghc-pkg init' to create the database with the correct format. Now, this is an old build tree, and I could blow it away entirely, but maybe there is something else going on. What should I do? Thanks (all on Linux) Simon