diff --git a/plugins/network/upnpc_ b/plugins/network/upnpc_
index 7e42fdea..c50037c6 100755
--- a/plugins/network/upnpc_
+++ b/plugins/network/upnpc_
@@ -28,7 +28,7 @@ Olivier Mehani <shtrom+munin@ssji.net>
 
 =head1 LICENSE
 
-GPLv2
+SPDX-License-Identifier: GPL-2.0-or-later
 
 =head1 MAGIC MARKERS
 
@@ -81,7 +81,8 @@ up.negative down
 EOF
 			;;
 		"traffic")
-			cat << EOF
+			# ${graph_period} is not a shell variable
+			cat << 'EOF'
 graph_title Uplink traffic
 graph_args --base 1024 -l 0
 graph_category network
@@ -97,7 +98,8 @@ up.negative down
 EOF
 			;;
 		"pkts")
-			cat << EOF
+			# ${graph_period} is not a shell variable
+			cat << 'EOF'
 graph_title Uplink packets
 graph_args --base 1000 -l 0
 graph_category network