pgbench error: (setshell) of script 0; execution of meta-command failed

Hi:

I run into the {subject} issue with the below setup.

cat foo.sql

\setshell txn_mode echo ${TXN_MODE}
\setshell speed echo ${SPEED}
\setshell sleep_ms echo ${SLEEP_MS}
\setshell subtxn_mode echo ${SUBTXN_MODE}

select 1;

$ TXN_MODE=-1 SPEED=1 SLEEP_MS=0 SUBTXN_MODE=-1 pgbench -n -ffoo.sql postgres -T5 -c4 --exit-on-abort

I *randomly*(7/8) get errors like:

pgbench (18devel)
pgbench: error: client 2 aborted in command 0 (setshell) of script 0; execution of meta-command failed
pgbench: error: Run was aborted due to an error in thread 0

I debug this for 1+ hour and didn't find anything useful, so I'd like
have a ask if there is any known issue or the way I use \setshell is
wrong?

Thanks

-- 
Best Regards
Andy Fan