Rotate shapes based on gradient

Hello,

I am a total expressions noob. I am trying to rotate a number of small shapes that rotate in waves with respect to a gradient layer behind it.

I've followed this tutorial which is what I want, but with scale instead of gradient:

https://www.youtube.com/watch?v=96ZM-s6yHEg

He uses the expression:

x = thisComp.layer("Gradient").sampleImage(transform.position, [.5,.5]/2, true, time)[0];
100*[x,x]

If I insert this into the Rotation field, I get the error "Expression must be of dimension 1, not 2". Where am I going wrong?

Many thanks!