diff --git a/src/activitypub/inbox.js b/src/activitypub/inbox.js
index a3a6648aa3..5e20d417f6 100644
--- a/src/activitypub/inbox.js
+++ b/src/activitypub/inbox.js
@@ -76,6 +76,22 @@ inbox.create = async (req) => {
 	}
 };
 
+inbox.add = async (req) => {
+	const { object, target } = req.body;
+
+	// Only react on Adds pertaining to local posts
+	const { type, id: pid } = await activitypub.helpers.resolveLocalId(object);
+	if (type === 'post') {
+		// Check context of OP
+		const tid = await posts.getPostField(pid, 'tid');
+		const context = await topics.getTopicField(tid, 'context');
+		if (context && context === target) {
+			activitypub.helpers.log(`[activitypub/inbox/add] Associating pid ${pid} with new context ${target}`);
+			await posts.setPostField(pid, 'context', target);
+		}
+	}
+};
+
 inbox.update = async (req) => {
 	const { actor, object } = req.body;
 	const isPublic = [...(object.to || []), ...(object.cc || [])].includes(activitypub._constants.publicAddress);
diff --git a/src/activitypub/mocks.js b/src/activitypub/mocks.js
index 09f6eeb934..5681425843 100644
--- a/src/activitypub/mocks.js
+++ b/src/activitypub/mocks.js
@@ -425,6 +425,9 @@ Mocks.notes.public = async (post) => {
 		return payload;
 	});
 
+	let context = await posts.getPostField(post.pid, 'context');
+	context = context || `${nconf.get('url')}/topic/${post.topic.tid}`;
+
 	const object = {
 		'@context': 'https://www.w3.org/ns/activitystreams',
 		id,
@@ -436,7 +439,7 @@ Mocks.notes.public = async (post) => {
 		updated,
 		url: id,
 		attributedTo: `${nconf.get('url')}/uid/${post.user.uid}`,
-		context: `${nconf.get('url')}/topic/${post.topic.tid}`,
+		context,
 		audience: `${nconf.get('url')}/category/${post.category.cid}`,
 		summary: null,
 		name,
diff --git a/src/api/activitypub.js b/src/api/activitypub.js
index 57870db800..b38f6bbc78 100644
--- a/src/api/activitypub.js
+++ b/src/api/activitypub.js
@@ -388,7 +388,7 @@ activitypubApi.add = enabledCheck((async (_, { pid }) => {
 
 	const tid = await posts.getPostField(localId || pid, 'tid');
 	const cid = await posts.getCidByPid(localId || pid);
-	if (cid <= 0) {
+	if (!utils.isNumber(tid) || cid <= 0) { // `Add` only federated on categorized topics started locally
 		return;
 	}