diff --git a/src/activitypub/mocks.js b/src/activitypub/mocks.js
index 5d827d35a4..7fc0958767 100644
--- a/src/activitypub/mocks.js
+++ b/src/activitypub/mocks.js
@@ -6,6 +6,7 @@ const path = require('path');
 const validator = require('validator');
 const sanitize = require('sanitize-html');
 
+const db = require('../database');
 const user = require('../user');
 const categories = require('../categories');
 const posts = require('../posts');
@@ -482,6 +483,14 @@ Mocks.notes.private = async ({ messageObj }) => {
 			`${nconf.get('url')}/api/v3/chats/${messageObj.roomId}/messages/${messageObj.toMid}` :
 			messageObj.toMid;
 	}
+	if (!inReplyTo) {
+		// Get immediately preceding message
+		const index = await db.sortedSetRank(`chat:room:${messageObj.roomId}:mids`, messageObj.mid);
+		const previousMid = await db.getSortedSetRange(`chat:room:${messageObj.roomId}:mids`, index - 1, index - 1);
+		inReplyTo = utils.isNumber(previousMid) ?
+			`${nconf.get('url')}/api/v3/chats/${messageObj.roomId}/messages/${previousMid}` :
+			messageObj.toMid;
+	}
 
 	const object = {
 		'@context': 'https://www.w3.org/ns/activitystreams',