"null is not an object" after doing checkout -b

[danmoseley]

starting with http://onlywei.github.io/explain-git-with-d3/#fetchrebase:

git fetch
Fetched 3 commits on master.
git rebase master
git rebase my-branch
Already up-to-date.
git checkout master
git checkout master
git commit
git rebase origin/master
git checkout my-branch
git commit
git rebase master
git checkout dccdc4d
git rebase my-branch
Cannot find commit: null
git checkout -b new
null is not an object (evaluating 'this.getCommit('HEAD').tags')