Merge on Raspberry Pi2 git server

Question

Hello Stack Overflow community,

I have configured a git server on my Raspberry PI 2 with the current Raspian Image (March 2018).

In my git repro I have two branches:

git@raspberrypi:~/database.git$ git branch
comments
*master

If I merge the 'comments' branch into the 'master' I get the following error message:

git@raspberrypi:~/database.git$ git merge comments
fatal: This operation must be run in a work tree

What am I doing wrong here?

Thank you,

Marc