diff --git a/app/assets/javascripts/discourse/models/nav-item.js.es6 b/app/assets/javascripts/discourse/models/nav-item.js.es6
index 6bc7ebc3d61..03cc8145c80 100644
--- a/app/assets/javascripts/discourse/models/nav-item.js.es6
+++ b/app/assets/javascripts/discourse/models/nav-item.js.es6
@@ -80,8 +80,9 @@ const NavItem = Discourse.Model.extend({
     }
   },
 
-  @computed("topicTrackingState", "name", "category")
-  count(state, name, category) {
+  @computed("name", "category", "topicTrackingState.messageCount")
+  count(name, category) {
+    const state = this.get("topicTrackingState");
     if (state) {
       return state.lookupCount(name, category);
     }
diff --git a/app/assets/javascripts/discourse/models/topic-tracking-state.js.es6 b/app/assets/javascripts/discourse/models/topic-tracking-state.js.es6
index 16513b74b10..d4c3a0354a3 100644
--- a/app/assets/javascripts/discourse/models/topic-tracking-state.js.es6
+++ b/app/assets/javascripts/discourse/models/topic-tracking-state.js.es6
@@ -179,8 +179,7 @@ const TopicTrackingState = Discourse.Model.extend({
     delete this.states["t" + topic_id];
   },
 
-  // If we have a cached topic list, we can update it from our tracking
-  // information.
+  // If we have a cached topic list, we can update it from our tracking information.
   updateTopics(topics) {
     if (Em.isEmpty(topics)) { return; }
 
@@ -283,7 +282,7 @@ const TopicTrackingState = Discourse.Model.extend({
   },
 
   incrementMessageCount() {
-    this.set("messageCount", this.get("messageCount") + 1);
+    this.incrementProperty("messageCount");
   },
 
   countNew(category_id) {
@@ -358,8 +357,7 @@ const TopicTrackingState = Discourse.Model.extend({
     const states = this.states;
     const idMap = Discourse.Category.idMap();
 
-    // I am taking some shortcuts here to avoid 500 gets for
-    // a large list
+    // I am taking some shortcuts here to avoid 500 gets for a large list
     if (data) {
       _.each(data,topic => {
         var category = idMap[topic.category_id];
diff --git a/test/javascripts/models/nav-item-test.js.es6 b/test/javascripts/models/nav-item-test.js.es6
index 15b75bc0dc6..38174eb639b 100644
--- a/test/javascripts/models/nav-item-test.js.es6
+++ b/test/javascripts/models/nav-item-test.js.es6
@@ -1,3 +1,5 @@
+import createStore from 'helpers/create-store';
+
 QUnit.module("Discourse.NavItem", {
   beforeEach() {
     Ember.run(function() {
@@ -19,3 +21,15 @@ QUnit.test('href', assert =>{
   href('category/bug', '/c/bug', 'English category name');
   href('category/确实是这样', '/c/343434-category', 'Chinese category name');
 });
+
+QUnit.test("count", assert => {
+  const navItem = createStore().createRecord("nav-item", { name: "new" });
+
+  assert.equal(navItem.get("count"), 0, "it has no count by default");
+
+  const tracker = navItem.get("topicTrackingState");
+  tracker.states["t1"] = { topic_id: 1, last_read_post_number: null };
+  tracker.incrementMessageCount();
+
+  assert.equal(navItem.get("count"), 1, "it updates when a new message arrives");
+});