[ODE] Camera should be a body with zero mass?

[andresch at cox.net]

The camera in my simulation is implemented as a body/geom in ode. I want my camera to follow my character as it moves and rotates (third person), so I attach them to each other with a slider, and all is well except...
the camera's mass affects the characters movement.  I want the camera affected by the guy, but not the guy affected by the camera, in other words the camera should be a body with zero mass.  If I set it's mass to zero, bad things happen so I'm thinking you're not supposed to do that. How can I have a body/geom that reacts to the things it's joined to but does not affect them?

-Andy