--- a/hgext/convert/cvsps.py	Sun Apr 19 12:20:39 2009 -0400
+++ b/hgext/convert/cvsps.py	Sun Apr 19 13:12:20 2009 -0400
@@ -637,7 +637,12 @@
                 m = m.group(1)
                 if m == 'HEAD':
                     m = None
-                candidate = changesets[branches[m]]
+                try:
+                    candidate = changesets[branches[m]]
+                except KeyError:
+                    ui.warn(_("warning: CVS commit message references "
+                              "non-existent branch %r:\n%s\n")
+                            % (m, c.comment))
                 if m in branches and c.branch != m and not candidate.synthetic:
                     c.parents.append(candidate)