Well, I don’t see how it could be a bug in Panda. Panda just passes the shader to the Cg compiler, (which is part of the NVIDIA Cg Toolkit).
The only thing I can think of is a bug in cgc, or the profile you’ve chosen doesn’t support it. If you’re on linux, typing “man cgc” at the console will list available profiles. This page lists all the profiles (but you probably have an older version of the Cg Toolkit).
If you’re on windows, try switching to the DirectX 9 driver.
You can also try googling to see if there are any other users with the same problem.
Or, it’s a bug in your code.