diff --git a/bin/tests/system/catz/tests.sh b/bin/tests/system/catz/tests.sh
index ffb1abd153..5197fbd94c 100644
--- a/bin/tests/system/catz/tests.sh
+++ b/bin/tests/system/catz/tests.sh
@@ -174,6 +174,56 @@ test -f ns2/zonedir/__catz___default_catalog1.example_dom1.example.db.jnl || ret
 if [ $ret -ne 0 ]; then echo_i "failed"; fi
 status=$((status+ret))
 
+n=$((n+1))
+echo_i "update catalog zone serial ($n)"
+ret=0
+# default minimum update rate is once / 5 seconds
+sleep 5
+$NSUPDATE -d <<END >> nsupdate.out.test$n 2>&1 || ret=1
+    server 10.53.0.1 ${PORT}
+    update add catalog1.example 3600 SOA . . 20 86400 3600 86400 3600
+    send
+END
+if [ $ret -ne 0 ]; then echo_i "failed"; fi
+status=$((status+ret))
+
+n=$((n+1))
+echo_i "wait for catalog zone to transfer ($n)"
+ret=0
+wait_for_soa_equal_20() {
+	dig_with_opts @10.53.0.2 SOA catalog1.example. > dig.out.test$n || return 1
+	grep "ANSWER: 1," dig.out.test$n > /dev/null || return 1
+	grep "status: NOERROR" dig.out.test$n > /dev/null || return 1
+	grep 'IN.SOA.\. \. 20 ' dig.out.test$n > /dev/null || return 1
+}
+retry_quiet 10 wait_for_soa_equal_20 || ret=1
+if [ $ret -ne 0 ]; then echo_i "failed"; fi
+status=$((status+ret))
+
+n=$((n+1))
+echo_i "update dom1.example. again ($n)"
+ret=0
+$NSUPDATE -d <<END >> nsupdate.out.test$n 2>&1 || ret=1
+   server 10.53.0.1 ${PORT}
+   update add foo.dom1.example 0 IN TXT added record
+   send
+END
+if [ $ret -ne 0 ]; then echo_i "failed"; fi
+status=$((status+ret))
+
+n=$((n+1))
+echo_i "wait for secondary to be updated again ($n)"
+ret=0
+wait_for_txt() {
+	dig_with_opts @10.53.0.2 TXT foo.dom1.example. > dig.out.test$n || return 1
+	grep "ANSWER: 1," dig.out.test$n > /dev/null || return 1
+	grep "status: NOERROR" dig.out.test$n > /dev/null || return 1
+	grep "IN.TXT." dig.out.test$n > /dev/null || return 1
+}
+retry_quiet 10 wait_for_txt || ret=1
+if [ $ret -ne 0 ]; then echo_i "failed"; fi
+status=$((status+ret))
+
 n=$((n+1))
 echo_i "removing domain dom1.example. from catalog1 zone ($n)"
 ret=0
diff --git a/lib/dns/catz.c b/lib/dns/catz.c
index b0c4527ee2..c519244b49 100644
--- a/lib/dns/catz.c
+++ b/lib/dns/catz.c
@@ -494,6 +494,14 @@ dns_catz_zones_merge(dns_catz_zone_t *target, dns_catz_zone_t *newzone) {
 					      zname, czname);
 			continue;
 		}
+
+		/*
+		 * Delete the old entry so that it won't accidentally be
+		 * removed as a non-existing entry below.
+		 */
+		dns_catz_entry_detach(target, &oentry);
+		result = isc_ht_delete(target->entries, key, (uint32_t)keysize);
+		RUNTIME_CHECK(result == ISC_R_SUCCESS);
 	}
 	RUNTIME_CHECK(result == ISC_R_NOMORE);
 	isc_ht_iter_destroy(&iter1);