Config doesn't work with Wallaby

[wsidl]

I'm going through the steps in the book "Learning OpenStack", but in the stage to download the config file and re-install devstack is failling with Wallaby.

wget .... -O local.conf
./stack.sh

gives:

+functions-common:git_clone:583                   git_update_remote_branch stable/wallaby
+functions-common:git_update_remote_branch:656   local git_branch=stable/wallaby
+functions-common:git_update_remote_branch:658   git checkout -b stable/wallaby -t origin/stable/wallaby
error: Your local changes to the following files would be overwritten by checkout:
             upper-constraints.txt
Please commit your changes or stash them before you switch branches.
Aborting
+functions-common:git_update_remote_branch:1   exit_trap

How do I fix this issue or is this an issue with the devstack installer?