diff --git a/templates/os-charm-phy-nic-mtu.conf b/templates/os-charm-phy-nic-mtu.conf
index edbb2588..7a471321 100644
--- a/templates/os-charm-phy-nic-mtu.conf
+++ b/templates/os-charm-phy-nic-mtu.conf
@@ -7,9 +7,9 @@ task
 script
     devs="{{ devs }}"
     mtu="{{ mtu }}"
-    tmpfile=`mktemp`
-    echo $devs > $tmpfile
     if [ -n "$mtu" ]; then
+        tmpfile=`mktemp`
+        echo $devs > $tmpfile
         while read -r dev; do
             [ -n "$dev" ] || continue
             rc=0