lib/isc/nothreads/include/isc/mutex.h:20:12: error: right-hand operand of comma expression has no effect

When building v9_11 with --disable-threads I get following warning:

gcc  -I/home/newman/isc/ws/bind9 -I../.. -I/home/newman/isc/ws/bind9/lib/lwres/include -I../../lib/lwres/unix/include -I../../lib/lwres/include -I/home/newman/isc/ws/bind9/lib/dns/include -I../../lib/dns/include -I/home/newman/isc/ws/bind9/lib/bind9/include -I../../lib/bind9/include -I/home/newman/isc/ws/bind9/lib/isc/include -I../../lib/isc -I../../lib/isc/include -I../../lib/isc/unix/include -I../../lib/isc/nothreads/include -I../../lib/isc/x86_32/include -I/home/newman/isc/ws/bind9/lib/isccfg/include -I../../lib/isccfg/include -I/usr/include    -DVERSION=\"9.11.21\" -DOPENSSL -DGSSAPI -DISC_LIST_CHECKINIT=1 -D_GNU_SOURCE -g -O2  -I/usr/include/libxml2    -fPIC  -W -Wall -Wmissing-prototypes -Wcast-qual -Wwrite-strings -Wformat -Wpointer-arith -fno-strict-aliasing -fno-delete-null-pointer-checks -Wshadow -Werror  \
	-DSESSION_KEYFILE=\"/var/run/named/session.key\" \
	-c ./nsupdate.c
In file included from /home/newman/isc/ws/bind9/lib/isc/include/isc/mem.h:22,
                 from ./nsupdate.c:34:
./nsupdate.c: In function ‘setup_system’:
../../lib/isc/nothreads/include/isc/mutex.h:20:12: error: right-hand operand of comma expression has no effect [-Werror=unused-value]
   20 |  (*(mp) = 0, ISC_R_SUCCESS)
      |  ~~~~~~~~~~^~~~~~~~~~~~~~~~
./nsupdate.c:1057:2: note: in expansion of macro ‘isc_mutex_init’
 1057 |  isc_mutex_init(&answer_lock);
      |  ^~~~~~~~~~~~~~
cc1: all warnings being treated as errors