diff -uNr a/tcp_fastopen_backup_key.sh b/tcp_fastopen_backup_key.sh
--- a/tcp_fastopen_backup_key.sh	2019-08-15 09:22:49.719175658 +0200
+++ b/tcp_fastopen_backup_key.sh	2019-08-15 09:23:09.271223084 +0200
@@ -30,7 +30,7 @@
 	ip netns exec "${NETNS}" ./tcp_fastopen_backup_key "$1"
 	val=$(ip netns exec "${NETNS}" nstat -az | \
 		grep TcpExtTCPFastOpenPassiveFail | awk '{print $2}')
-	if [ $val -ne 0 ]; then
+	if [ "$val" -ne 0 ]; then
 		echo "FAIL: TcpExtTCPFastOpenPassiveFail non-zero"
 		return 1
 	fi