diff --git a/src/categories/index.js b/src/categories/index.js
index b201e8e204..cbc99e3c79 100644
--- a/src/categories/index.js
+++ b/src/categories/index.js
@@ -140,6 +140,7 @@ Categories.getModerators = function (cid, callback) {
 
 Categories.getModeratorUids = function (cids, callback) {
 	var sets;
+	var uniqGroups;
 	async.waterfall([
 		function (next) {
 			var groupNames = cids.reduce(function (memo, cid) {
@@ -162,11 +163,13 @@ Categories.getModeratorUids = function (cids, callback) {
 				return memo;
 			}, { groupNames: [], uids: [] });
 
-			groups.getMembersOfGroups(sets.groupNames, next);
+			uniqGroups = _.uniq(_.flatten(sets.groupNames));
+			groups.getMembersOfGroups(uniqGroups, next);
 		},
 		function (groupUids, next) {
+			var map = _.zipObject(uniqGroups, groupUids);
 			const moderatorUids = cids.map(function (cid, index) {
-				return _.union(sets.uids[index].concat(groupUids[index]));
+				return _.uniq(sets.uids[index].concat(_.flatten(sets.groupNames[index].map(g => map[g]))));
 			});
 			next(null, moderatorUids);
 		},
diff --git a/test/categories.js b/test/categories.js
index 30e5c30842..644ce6adb1 100644
--- a/test/categories.js
+++ b/test/categories.js
@@ -856,10 +856,27 @@ describe('Categories', function () {
 				});
 			});
 
+			it('should not fail when there are multiple groups', function (done) {
+				async.series([
+					async.apply(groups.create, { name: 'testGroup2' }),
+					async.apply(groups.join, 'cid:1:privileges:groups:moderate', 'testGroup2'),
+					async.apply(groups.join, 'testGroup2', 1),
+					function (next) {
+						Categories.getModeratorUids([1, 2], function (err, uids) {
+							assert.ifError(err);
+							assert(uids[0].includes('1'));
+							next();
+						});
+					},
+				], done);
+			});
+
 			after(function (done) {
 				async.series([
 					async.apply(groups.leave, 'cid:1:privileges:groups:moderate', 'testGroup'),
+					async.apply(groups.leave, 'cid:1:privileges:groups:moderate', 'testGroup2'),
 					async.apply(groups.destroy, 'testGroup'),
+					async.apply(groups.destroy, 'testGroup2'),
 				], done);
 			});
 		});