fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.

Hello,

        (s017241) [/usr/src/linux-2.6] git checkout v2.6.20
        warning: You appear to be on a branch yet to be born.
        warning: Forcing checkout of v2.6.20.
        fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
        Use '--' to separate paths from revisions
        Cannot detach HEAD

(s017241) [/usr/src/linux-2.6] git version
git version 1.5.2.1

what is going wrong here?

        Thomas