RenderSettings.ambientProbe "empty", how to access?

I have ambient based on skybox set up in my scene, with no other form of GI enabled. When I retrieve the SphericalHarmonicsL2 from ambientProbe, coefficient 0 is white, and all other coefficients appear to be black. (The ambient lighting is working correctly in the scene, I am just trying to access the color data in my script.) Is there only a certain point in the rendering process when this data is accessible? Am I just misunderstanding how this data structure works? Anybody have any success grabbing the skybox ambient light into a script?