Real bastard that error.
Are you on Max6 or Max7?
Has it ways happened throughout whatever betas you've tried?
(Are you on beta5?)
I've made tons of changes to the code (though not the core monomeControl stuff) as it's kind of closer to TPV2 now, and I'm currently away from my monome (the 128 anyways) so I can't try figuring other ways around it, but I might just try to sum those two lists "manually" to see if that changes anything.
Actually, try this out. I have basically taken the vexpr that I remember throwing up the errors and removed it, instead I'm manually unpacking the lists, summing the individual components, and then repacking the lists. Pretty awful/clunky/sloppy, but it might work. Either way it's worth testing (I can't at the moment).
If this works I'll tidy it up and push it forward into the more recent version.
monomeControl.maxpat (530.1 KB)