Github user MikeThomsen commented on the issue:
@JohannesDaniel see my earlier comment about rebasing. You **cannot** just
do `git merge master` into another branch like that since master is the
foundation of that branch. Don't worry, though. The merge commit should get
stomped when you do the rebase and forced push.
Give that a shot. No need to close out the PR over some minor learning
pains with Git. Post a message when you've force pushed and I'll take a look.