Can't find svn parent of a merge revision

I want to use mercurial on my development maschine and connect to an svn server. I can import and pull/push updates. But, when I merge a feature branch to default (aka trunk) and want to push the code I always get the error message "Sorry, can't find svn parent of a merge revision."
Is it possible to use branches and merge them back and then pull to the svn server? What is the correct workflow? Do I have to rebase? If so, when?

https://bitbucket.org/durin42/dotfiles/src/05ccb6c5fb4b9df88e06bdc9d162912760eb554e/.shell.d/50.hg_functions.sh?at=default#cl-46 is how I did this at my last company. It's a pain, but works.

This is just not something that'll get worked on for now, because we can't pull the merges back from the server effectively.