diff --git a/downstream.go b/downstream.go
index 7d0e5c8..be2faf8 100644
--- a/downstream.go
+++ b/downstream.go
@@ -2473,6 +2473,9 @@ func (dc *downstreamConn) handleMessageRegistered(ctx context.Context, msg *irc.
 		if uc == nil {
 			return newUnknownCommandError(msg.Command)
 		}
+		if _, ok := uc.isupport["MONITOR"]; !ok {
+			return newUnknownCommandError(msg.Command)
+		}
 
 		var subcommand string
 		if err := parseMessageParams(msg, &subcommand); err != nil {
diff --git a/upstream.go b/upstream.go
index 218cd4d..c4d50e0 100644
--- a/upstream.go
+++ b/upstream.go
@@ -2075,6 +2075,10 @@ func (uc *upstreamConn) updateChannelAutoDetach(name string) {
 }
 
 func (uc *upstreamConn) updateMonitor() {
+	if _, ok := uc.isupport["MONITOR"]; !ok {
+		return
+	}
+
 	add := make(map[string]struct{})
 	var addList []string
 	seen := make(map[string]struct{})