Odd errors on llSetPrimitiveParams

Hello,
I am taking two lists produced with llGetPrimitiveParams using the PRIM_TYPE querry, combining about half of one with half of the other, and trying to use the result to set the primitive params on another object. I start out using two identicle lists, from unmodified cubes. They are sent to another object. The script in the other object "combines" the lists, but because the originals are identicle this has no effect and the resulting list is just like the original list. I can't figure out weather to prepend PRIM_TYPE to the input on llSetPrimitiveParams. No matter what I do, it gives me errors about the input list being malformed, when it seems identicle to what was read from the other objects. It says it needs an integer primitive type, and then I try to fix that and it complains about a non-integer perameter or something. Has anyone done anything like this before? Do you have any tips? Or solutions? Or working examples?
Thanks,
Interfect

easier to get an answer if you post a script example so we can test out in world or at least know the logic you set behind it.