https://github.com/jbevain/cecil/blob/master/Mono.Cecil/BaseAssemblyResolver.cs#L369 hardcodes "2" as the number of entries in gac_paths, but if you're doing something as dumb as I'm doing, in GetGacPaths here https://github.com/jbevain/cecil/blob/master/Mono.Cecil/BaseAssemblyResolver.cs#L291 if you're not on mono AND you don't have the WINDIR env var, you'll end up with an empty gac paths collection and blow up in the lookup.