Non uniform scaling screws up shading in Coin3D

Hi,

I have a simple scene with a triangulated surface that contains a
SoTransform node. I use the transform node only for scaling the scene.
As long as the transformation is uniform or nearly uniform, the shading
looks fine, but when I set a strongly non uniform shading the surface
becomes very dark except for the direct reflection.

My transformation matrix looks like
[ 1 0 0 0
  0 1 0 0
  0 0 z 0
  0 0 0 1]

I get good results for 0.7 < z < 2 and a pitch black surface
for z > 1000.

Is this a bug in Coin or maybe a floating point precission problem?
I allready tried to explicitly supply an SoNormal node, but that did
not fix the problem.

Thanks for any hints on this!

Georg