diff --git a/debian/grep_dd_sigs b/debian/grep_dd_sigs
index 438723c..f7e1ac7 100755
--- a/debian/grep_dd_sigs
+++ b/debian/grep_dd_sigs
@@ -15,6 +15,15 @@
# You should have received a copy of the GNU General Public License
# along with this program. If not, see .
+if [ $# -ne 1 ] ; then
+ echo "Usage:"
+ echo " $0 "
+ echo " can be either string that identifies a GPG key: key"
+ echo "ID, fingerprint, UID, etc. (see 'HOW TO SPECIFY A USER ID' in"
+ echo "the gpg's manpage)."
+ exit 1
+fi
+
sigs=$(gpg --with-colons --check-sigs $1 \
| grep "sig:\!" \
| sed -r "s/sig:\!::[[:digit:]]+:([[:alnum:]]+):.*$/\1/")