iptables -Z option

[Bonno Bloksma <b.bloksma@tio.nl>]

Hi,

For years I have had a firewall script the sets and/or resets my firewall rules. Is starts of near the top with some lines that have been there for ever.
After upgrading to buster I got an error executing this script and I cannot find out why. Using the extra echo lines I have been able to pinpoint the error to the iptables -Z line

[...]
IPTABLES=/usr/sbin/iptables
echo flush
# Flush all rules in all chains and then delete all chains
chains=`cat /proc/net/ip_tables_names 2>/dev/null`
for i in $chains; do $IPTABLES -t $i -F; done
for i in $chains; do $IPTABLES -t $i -X; done
echo counters
# Reset all counters for default chains
$IPTABLES -Z
echo "return traffic"
[...]

This will produce the following output.
flush
counters
iptables v1.8.2 (nf_tables):  RULE_REPLACE failed (Invalid argument): rule in chain INPUT
return traffic

Can anyone tell me why the re reset counter line fails with a reference to the INPUT chain? There is loads of documentation about iptables but nothing about the -Z option.
I have my iptables rules in a separate script that I can test and if I ever shut myself out I can simply restart the machine and the default / previous ruleset will load and all will be up and running again. I'd like to keep that way of setting things up, it makes it easy to test a new set of rules and debug typo's.

Bonno Bloksma