I got the following error message:
Stackoverflow helped, as usual. As I am the sole developer of my can of worms, I just added the '-f' flag. It's nice to do the last commit of the photometry code which I've been working on for the last 3 months.
To prevent you from losing history, non-fast-forward updates were rejected Merge the remote changes before pushing again. See the 'Note about fast-forwards' section of 'git push --help' for details.