Expression result must be of dimension 2 not 1

It's hard to tell from your screen shot, but do you have the whole expression? The part that's visible ends with connecting the controller variable to the slider. If you end the expression there, you will indeed get that error message because controller is one-dimensional. If you add the rest of the expression (or even just the next line), you should end up with a 2-dimensional value and the error should disappear. Hope that helps.