diff --git a/ssh_filter_btrbk.sh b/ssh_filter_btrbk.sh
old mode 100644
new mode 100755
index ed24614..9e5746d
--- a/ssh_filter_btrbk.sh
+++ b/ssh_filter_btrbk.sh
@@ -1,8 +1,12 @@
 #!/bin/sh
 
+set -e
+set -u
+
 export PATH=/sbin:/bin:/usr/sbin:/usr/bin
 
-if [ "$1" = "-l" ]; then
+enable_log=
+if [ "$#" -ge 1 ] && [ "$1" = "-l" ]; then
     enable_log=1
 fi