diff --git a/src/categories/activeusers.js b/src/categories/activeusers.js
index 40ff7bc016..726f612a6a 100644
--- a/src/categories/activeusers.js
+++ b/src/categories/activeusers.js
@@ -7,10 +7,13 @@ var posts = require('../posts');
 var db = require('../database');
 
 module.exports = function (Categories) {
-	Categories.getActiveUsers = function (cid, callback) {
+	Categories.getActiveUsers = function (cids, callback) {
+		if (!Array.isArray(cids)) {
+			cids = [cids];
+		}
 		async.waterfall([
 			function (next) {
-				db.getSortedSetRevRange('cid:' + cid + ':pids', 0, 24, next);
+				db.getSortedSetRevRange(cids.map(cid => 'cid:' + cid + ':pids'), 0, 24, next);
 			},
 			function (pids, next) {
 				posts.getPostsFields(pids, ['uid'], next);