diff --git a/hgext/convert/cvsps.py b/hgext/convert/cvsps.py
--- a/hgext/convert/cvsps.py
+++ b/hgext/convert/cvsps.py
@@ -637,7 +637,12 @@ def createchangeset(ui, log, fuzz=60, me
                 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)
 
diff --git a/tests/test-convert-cvs-detectmerge b/tests/test-convert-cvs-detectmerge
--- a/tests/test-convert-cvs-detectmerge
+++ b/tests/test-convert-cvs-detectmerge
@@ -31,8 +31,10 @@ filterpath()
 
 echo "[extensions]" >> $HGRCPATH
 echo "convert = " >> $HGRCPATH
+echo "graphlog = " >> $HGRCPATH
 echo "[convert]" >> $HGRCPATH
 echo "cvsps=builtin" >> $HGRCPATH
+echo "cvsps.cache=0" >> $HGRCPATH
 echo "cvsps.mergefrom=\[MERGE from (\S+)\]" >> $HGRCPATH
 
 echo % create cvs repository with one project
@@ -90,6 +92,18 @@ filterpath cvscall -Q update -j1.2 -j1.3
 cvsci -m"add other text [MERGE from HEAD]" file1
 
 set -e
+echo "% fix bug on v1_1, merge to trunk with error"
+cvscall -Q update -rv1_1
+echo "merge forward" >> file1
+cvscall -Q tag unmerged
+cvsci -m"fix file1"
+cvscall -Q update -A
+filterpath cvscall -Q update -junmerged -jv1_1
+# note the typo in the commit log message
+cvsci -m"fix file1 [MERGE from v1-1]"
+cvs -Q tag -d unmerged
+
+set -e
 echo % convert to hg
 cd ..
 filterpath hg convert proj proj.hg
@@ -98,11 +112,5 @@ echo % complete log
 template="{rev}: '{branches}' {desc}\n"
 hg -R proj.hg log --template="$template"
 
-echo % parents of rev 3
-hg -R proj.hg parents --template="$template" -r3
-echo % parents of rev 4
-hg -R proj.hg parents --template="$template" -r4
-echo % parents of rev 5
-hg -R proj.hg parents --template="$template" -r5
-echo % parents of rev 7
-hg -R proj.hg parents --template="$template" -r7
+echo % graphical log
+hg -R proj.hg glog --template="$template"
diff --git a/tests/test-convert-cvs-detectmerge.out b/tests/test-convert-cvs-detectmerge.out
--- a/tests/test-convert-cvs-detectmerge.out
+++ b/tests/test-convert-cvs-detectmerge.out
@@ -60,26 +60,43 @@ retrieving revision 1.2
 retrieving revision 1.3
 Merging differences between 1.2 and 1.3 into file1
 cvs -f ci -madd other text [MERGE from HEAD] file1
+% fix bug on v1_1, merge to trunk with error
+cvs -f -Q update -rv1_1
+cvs -f -Q tag unmerged
+cvs -f ci -mfix file1
+cvs -f -Q update -A
+cvs -f -Q update -junmerged -jv1_1
+RCS file: *REPO*/proj/file1,v
+retrieving revision 1.1.4.2
+retrieving revision 1.1.4.3
+Merging differences between 1.1.4.2 and 1.1.4.3 into file1
+cvs -f ci -mfix file1 [MERGE from v1-1]
 % convert to hg
+warning: CVS commit message references non-existent branch 'v1-1':
+fix file1 [MERGE from v1-1]
 initializing destination proj.hg repository
 connecting to *REPO*
 scanning source...
 using builtin cvsps
 collecting CVS rlog
-10 log entries
+12 log entries
 creating changesets
-8 changeset entries
+10 changeset entries
 sorting...
 converting...
-7 add file1 on trunk
-6 add text
-5 unrelated change
-4 add text [MERGE from v1_0]
-3 add text [MERGE from v1_1]
-2 add file2 on trunk
-1 add other text
-0 add other text [MERGE from HEAD]
+9 add file1 on trunk
+8 add text
+7 unrelated change
+6 add text [MERGE from v1_0]
+5 add text [MERGE from v1_1]
+4 add file2 on trunk
+3 add other text
+2 add other text [MERGE from HEAD]
+1 fix file1
+0 fix file1 [MERGE from v1-1]
 % complete log
+9: '' fix file1 [MERGE from v1-1]
+8: 'v1_1' fix file1
 7: 'v1_1' add other text [MERGE from HEAD]
 6: '' add other text
 5: '' add file2 on trunk
@@ -88,14 +105,24 @@ 3: 'v1_1' add text [MERGE from v1_0]
 2: 'v1_1' unrelated change
 1: 'v1_0' add text
 0: '' add file1 on trunk
-% parents of rev 3
-2: 'v1_1' unrelated change
-1: 'v1_0' add text
-% parents of rev 4
-0: '' add file1 on trunk
-3: 'v1_1' add text [MERGE from v1_0]
-% parents of rev 5
-4: '' add text [MERGE from v1_1]
-% parents of rev 7
-3: 'v1_1' add text [MERGE from v1_0]
-6: '' add other text
+% graphical log
+o  9: '' fix file1 [MERGE from v1-1]
+|
+| o  8: 'v1_1' fix file1
+| |
+| o  7: 'v1_1' add other text [MERGE from HEAD]
+|/|
+o |  6: '' add other text
+| |
+o |  5: '' add file2 on trunk
+| |
+o |  4: '' add text [MERGE from v1_1]
+|\|
+| o    3: 'v1_1' add text [MERGE from v1_0]
+| |\
++---o  2: 'v1_1' unrelated change
+| |
+| o  1: 'v1_0' add text
+|/
+o  0: '' add file1 on trunk
+