Setting an array or group parameter to an expression results in an invalid expression.
Steps to reproduce
1) Open Katana and create a Material node
2) Add any (prman/arnold) surface shader
3) Right-Click on the colour parameter and click on Expression. *Note: Error message spews in terminal (see below)*
4) Expand the colour parameter: the single R, G, B values should have disappeared. Running
print NodegraphAPI.GetNode('Material').getParameter('shaders.arnoldSurfaceParams.Kd_color.value')
in the Python console prints None.
Workaround
Instead of setting an expression on the whole group or array parameter, set expressions on the individual child parameters.
Reproduced by support
Katana 1.6v3, 2.0v1, 2.0v5, 2.1v1, 2.1v5, 2.5v1, 2.5v4 RHEL 6.3
Katana 2.5v1, 2.5v4 Windows 7