If my memory serves me right, its because the alpha from a vector4 is a seperate output, while in the collection param its embedded in the output.
Try and append the alpha with the rgb output from the vector4 and you’ll get the same error.
You can mask out the alpha part of the collection parameter, or perhaps (and havent tried it… so dont take my word on it) try and append a 0 or 1 into the absolute world position and it should take the collection parameter as is.