Ordinarily I wouldn\'t just post an error message on SO, but after a Google search only found one hit, I thought I\'d at least open the floor for this error here on SO.
It's been an eventful 27 minutes... :)
Changing the dependency property's type from unit[]
to IList
solved the problem. Best of all, it didn't requite many code changes as the array already implements that interface.
I'm not sure whether dispatching to the array via the interface (callvirt) is slower. My guess is yes.
The original error message hints that there's something going on here that I don't quite understand. I'll accept any answer that explains it properly.