diff --git a/CHANGES b/CHANGES
index 32ebaec2f9..ddc71de48f 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,5 @@
+6337.	[bug]		Nsupdate could assert while shutting down. [GL #4529]
+
 6336.	[func]		Expose the zones with the 'first refresh' flag set in
 			statistics channel's "Incoming Zone Transfers" section
 			to indicate the zones that are not yet fully ready, and
diff --git a/lib/dns/request.c b/lib/dns/request.c
index 14a96058c2..400cfe04f5 100644
--- a/lib/dns/request.c
+++ b/lib/dns/request.c
@@ -190,15 +190,20 @@ requests_shutdown(void *arg) {
 
 void
 dns_requestmgr_shutdown(dns_requestmgr_t *requestmgr) {
+	bool first;
 	REQUIRE(VALID_REQUESTMGR(requestmgr));
 
 	req_log(ISC_LOG_DEBUG(3), "%s: %p", __func__, requestmgr);
 
 	rcu_read_lock();
-	INSIST(atomic_compare_exchange_strong(&requestmgr->shuttingdown,
-					      &(bool){ false }, true));
+	first = atomic_compare_exchange_strong(&requestmgr->shuttingdown,
+					       &(bool){ false }, true);
 	rcu_read_unlock();
 
+	if (!first) {
+		return;
+	}
+
 	/*
 	 * Wait until all dns_request_create{raw}() are finished, so
 	 * there will be no new requests added to the lists.