recipe to merge in specific git branch is instead merging HEAD

I've created a recipe to take the standard Ubuntu package source for a package, merge in my own branch and then build it.

https://code.launchpad.net/~brightbox/+recipe/nginx-healthcheck

The upstream branch it uses is: https://code.launchpad.net/~usd-import-team/ubuntu/+source/nginx/+git/nginx/+ref/ubuntu/xenial-security

My own branch is in the git repository https://code.launchpad.net/~brightbox/nginx-healthcheck/+git/nginx-healthcheck and is named "xenial-healthcheck"

The recipe line specifies the repository and the branch as the docs say:

merge xenial-healthcheck lp:nginx-healthcheck xenial-healthcheck

but when this recipe is run, it tries to fetch a branch named HEAD rather than xenial-healthcheck, which fails because there is no branch or object named HEAD:

https://launchpadlibrarian.net/331376898/buildlog.txt.gz

The command that git-build-recipe runs is:

git fetch lp:nginx-healthcheck HEAD:refs/remotes/source-xenial-healthcheck/xenial-healthcheck refs/heads/*:refs/remotes/source-xenial-healthcheck/*

which if I run locally, outputs:

fatal: Couldn't find remote ref HEAD

I created a branch named HEAD as a duplicate of xenial-healthcheck and now the recipe builds ok.

How do I get launchpad to use the xenial-healthcheck branch and not HEAD? I want to have several recipes using different branches from this git repository, so I can't just use HEAD.

The docs do state that you specify the branch name as the third parameter to the merge command:

https://help.launchpad.net/Packaging/SourceBuilds/Recipes#Merging

Any pointers?