git subtree woes

jchase@tethys testing $ git init .
Initialized empty Git repository in /tmp/testing/.git/
jchase@tethys testing $ git commit -m "root" --allow-empty
[master (root-commit) 1c17895] root
jchase@tethys testing $ mkdir sub/tree -p
jchase@tethys testing $ touch sub/this
tjchase@tethys testing $ touch sub/tree/that
jchase@tethys testing $ git add sub/this
jchase@tethys testing $ git commit -m "this"
[master ebb2e3c] this
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 sub/this
jchase@tethys testing $ git add sub/tree/that
jchase@tethys testing $ git commit -m "that"
[master 0ea4e02] that
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 sub/tree/that
jchase@tethys testing $ git subtree split --prefix sub --branch subtree
Created branch 'subtree'
e586a79d441cd086e9bd9287353857e9d7f1dcdc
jchase@tethys testing $ git checkout subtree
Switched to branch 'subtree'
jchase@tethys testing $ mkdir tree/branch
jchase@tethys testing $ touch tree/branch/leaf
jchase@tethys testing $ git add tree/branch/leaf
jchase@tethys testing $ git commit -m "leaf"
[subtree e612d63] leaf
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 tree/branch/leaf
jchase@tethys testing $ git checkout master
Switched to branch 'master'
jchase@tethys testing $ git subtree merge -P sub subtree
fatal: refusing to merge unrelated histories