git checkout --theirs fails

I'm doing a rebase and got some conflicts. I just want to take their version of all files, but git checkout --theirs complains:

--ours/--theirs' cannot be used with switching branches

What gives? I'm not *trying* to switch branches. I just want to resolve the conflict by taking their version. If I try git checkout --theirs ., then it complains that not every single file in the directory has a "their" version. So? Take the ones that do.