How to create a new branch based on a tag?

Hi,

   I would like to create a branch of my own work based on a specific 
kernel version(i.e
v2.6.11). I tried with:

 > git-branch my2.6.11 v2.6.11
error: Object 5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c is a tree, not a 
commit
fatal: Needed a single revision

  As v2.6.11 is a tree object(tag), git-checkout and git-branch could 
not process it. The
only way I know is using git-read-tree to fetch v2.6.11 content into 
current index and start
a branch based on it. Is there any simpler method?

Thanks!