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?

 

Working

Error 192 (sglib): Conflicts or change in the working co…