diff --git a/cvsup/cvsup b/cvsup/cvsup
index a88ca1c..c38edc4 100755
--- a/cvsup/cvsup
+++ b/cvsup/cvsup
@@ -8,6 +8,10 @@ die() { echo "$*" >&2; exit 1; }
 export LOCKFILE=${HOME}/.cvsup/$1_lock
 export DATEFILE=${HOME}/.cvsup/$1_date
 
+if ( [ ${#} -ne 2 ] ); then
+	die "Usage: cvsup <tree> <branch>\n"
+fi
+
 if ( [ -e $LOCKFILE ]); then
 	LOCKPID=`cat $LOCKFILE`
 	if ( ! ps -p $LOCKPID|egrep -q "$LOCKPID" ); then