Why does my vertical impulse in ofxBox2d results in rotation?

I’m using the ofxBox2d plugin in of 0.9 for my physics. As a start, I want a cube to jump. Without any additional velocities applied, this should result in a movement on the vertical axis.

My objects’ physics:
box.setPhysics(10,0,0);

The impulse used for jumping:
box.addImpulseForce(ofVec2f(0,-5), box.getPosition());

Unfortunately, the impulse doesn’t seem to be applied at the object’s centre of mass, resulting in a crazy rotation. On http://www.iforce2d.net/b2dtut/jumping , it says to pass box.GetWorldCenter() as a second argument, but this is not available in ofxBox2d. I tried to fetch this information from box.body->getWorldCentre(), but these values are not very helpful.

Any ideas on how to fix this?