Error 192 Conflicts or changes in the working copy prevented update

I have this error which happened after I tried to do a few simple merges.
> vv diff
(Empty response; I assume that the working copy is clean)
> vv branch
cm2 (This is a branch I've just successfully committed a merge to)
> vv update -b b
vv.exe: Error 192 (sglib): Conflicts or changes in the working directory prevented UPDATE:
There are uncommitted changes in the working directory and the goal change set is not a descendant or ancestor of the current baseline.
How do I prevent this from happening?
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Working
Error 192 (sglib): Conflicts or change in the working co…
