Help - getting thrown out of my vehicle ...

I hope someone here can shed some light on a problem I'm having. I have a vehicle with a number of linked child parts, one of these parts has a script to respond to llMessageLinked which uses llSetPrimitiveParams to change the shape of that part (specifically, alter the dimple).
The problem I have is that if I am sitting in the vehicle when the part gets the message I am thrown out.

At first I thought this was because the shape changed caused it to intersect my avatar, but I've moved it out of the way (and also tried making it a phantom) but it still happens.

If I don't send the llMessageLinked the vehicle works as intended.

Have I missed an important limitation of something-or-other in all this - any clues? I've been pulling my hair out trying to get this working!

llSetPrimativeParams unseats any avatar in the link set. Lindens added this the first patch after they gave us SetPrimativeParamaters, to fix a crash issue, and I guess they've never fixed it any further.

Excellent, an answer - many thanks.

I guess my vehicle just wont be able to change shape quite as I'd originally imagined. Turning from a balloon to a boat at 15m up is fun though, shame I can't change the shape of the vehicle without losing the passenger on the ride down.