TheFluff: Thanks for your help. I think it is the Avisynth script parser as using vanilla ScriptClip or FrameEvaluate gives me that error.
Code:
Assert(!(yPlane==uPlane==vPlane==-1), "You need to select at least one plane")
I was also not sure this would work but it seemed to work just fine.
Thank you, StainlessS, your code is definitely a better alternative than what I came up with.