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.