diff --git a/git-rebase.sh b/git-rebase.sh
index 6ec155c..6503113 100755
--- a/git-rebase.sh
+++ b/git-rebase.sh
@@ -34,6 +34,8 @@
 require_work_tree
 cd_to_toplevel
 
+LF='
+'
 OK_TO_SKIP_PRE_REBASE=
 RESOLVEMSG="
 When you have resolved this problem run \"git rebase --continue\".
@@ -417,7 +419,22 @@
 
 # Make sure the branch to rebase onto is valid.
 onto_name=${newbase-"$upstream_name"}
-onto=$(git rev-parse --verify "${onto_name}^0") || exit
+if	left=$(expr "$onto_name" : '\(.*\)\.\.\.') &&
+	right=$(expr "$onto_name" : '\.\.\.\(.*\)$') &&
+	: ${left:=HEAD} ${right:=HEAD} &&
+	onto=$(git merge-base "$left" "$right")
+then
+	case "$onto" in
+	?*"$LF"?*)
+		die "$onto_name: there are more than one merge bases"
+		;;
+	'')
+		die "$onto_name: there is no merge base"
+		;;
+	esac
+else
+	onto=$(git rev-parse --verify "${onto_name}^0") || exit
+fi
 
 # If a hook exists, give it a chance to interrupt
 run_pre_rebase_hook "$upstream_arg" "$@"