diff --git a/shellpki b/shellpki
index f04e552..e6b9b00 100755
--- a/shellpki
+++ b/shellpki
@@ -763,7 +763,7 @@ list() {
         exit 0
     fi
 
-    if [ -z "${1}" ]; then
+    if [ -z "${1:-}" ]; then
         show_usage >&2
         exit 1
     fi
@@ -772,7 +772,7 @@ list() {
     list_revoked=1
 
     while :; do
-        case "${1}" in
+        case "${1:-}" in
             -a|--all)
                 list_valid=0
                 list_revoked=0