If you don’t mind, could you please read the following post and then edit your post to match its recommendations:
Specifically, having a MWE of what you’re trying to do would be very helpful. All I know is that a return type of Union{} means that your kernel throws an error somewhere, but without an MWE I couldn’t tell you where that is.
Also, CLArrays is broken right now; what OS and GPU are you trying to do GPGPU computing with?