diff --git a/src/user/categories.js b/src/user/categories.js
index 137cf595e3..38770138a9 100644
--- a/src/user/categories.js
+++ b/src/user/categories.js
@@ -4,19 +4,25 @@ const _ = require('lodash');
 
 const db = require('../database');
 const categories = require('../categories');
-const activitypub = require('../activitypub');
 const plugins = require('../plugins');
+const utils = require('../utils');
 
 module.exports = function (User) {
 	User.setCategoryWatchState = async function (uid, cids, state) {
-		if (!activitypub.helpers.isUri(uid) && !(parseInt(uid, 10) > 0)) {
+		if (utils.isNumber(uid) && parseInt(uid, 10) <= 0) {
 			return;
 		}
+
 		const isStateValid = Object.values(categories.watchStates).includes(parseInt(state, 10));
 		if (!isStateValid) {
 			throw new Error('[[error:invalid-watch-state]]');
 		}
-		cids = Array.isArray(cids) ? cids : [cids];
+
+		cids = new Set(Array.isArray(cids) ? cids : [cids]);
+		cids.delete(-1); // cannot watch cid -1
+		cids.delete('-1');
+		cids = Array.from(cids);
+
 		const exists = await categories.exists(cids);
 		if (exists.includes(false)) {
 			throw new Error('[[error:no-category]]');