Fix #2738

src/misc/should-mute-this-note.ts

+import * as mongo from 'mongodb';
+
+function toString(id: any) {
+	return mongo.ObjectID.prototype.isPrototypeOf(id) ? (id as mongo.ObjectID).toHexString() : id;
+}
+
 export default function(note: any, mutedUserIds: string[]): boolean {
-	if (mutedUserIds.indexOf(note.userId) != -1) {
+	if (mutedUserIds.includes(toString(note.userId))) {
 		return true;
 	}
 
-	if (note.reply != null && mutedUserIds.indexOf(note.reply.userId) != -1) {
+	if (note.reply != null && mutedUserIds.includes(toString(note.reply.userId))) {
 		return true;
 	}
 
-	if (note.renote != null && mutedUserIds.indexOf(note.renote.userId) != -1) {
+	if (note.renote != null && mutedUserIds.includes(toString(note.renote.userId))) {
 		return true;
 	}