Fixing a merge conflict in NGit with theirs?

Question

I've been looking around and there's not a lot of clear examples as to how to resolve this. I found this example on stack for JGit, but it doesn't appear to be working for NGIT.

I've done:

var merge = NG.Merge.MergeStrategy.THEIRS.NewMerger(localRepo);
var obj = new NG.ObjectId[] { localRepo.Resolve("a"), localRepo.Resolve("c") };
merge.Merge(obj);

However "a" and "c" resolve to null. So unsurprisingly, the merge fails with a null reference exception.

Any help here would be appreciated!