diff --git a/plugins/zfs/zpool_iostat b/plugins/zfs/zpool_iostat
index 4d6a41d6..faf7deb9 100755
--- a/plugins/zfs/zpool_iostat
+++ b/plugins/zfs/zpool_iostat
@@ -1,11 +1,18 @@
 #!/bin/sh
+# -*- sh -*-
 
+set -eu
+
+
+# shellcheck source=/usr/share/munin/plugins/plugin.sh
 . "$MUNIN_LIBDIR/plugins/plugin.sh"
 
+
 ZPOOL_BIN=/sbin/zpool
+ACTION="${1:-}"
 
 
-if [ "$1" = "autoconf" ]; then
+if [ "$ACTION" = "autoconf" ]; then
 	if [ -x "$ZPOOL_BIN" ]; then
 		echo yes
 	else
@@ -27,7 +34,7 @@ get_pool_iostat() {
 		| gawk '/M/ {print strtonum($1)*1000}; /K/ {print strtonum($1)}; /[0-9]$/ {print int($1)/1000}'
 }
 
-if [ "$1" = "config" ]; then
+if [ "$ACTION" = "config" ]; then
         echo 'graph_title zpool iostat'
         echo 'graph_args --base 1000 -l 0'
         echo 'graph_vlabel write - read KBytes/s'