diff --git a/backend/jx_helpers.js b/backend/jx_helpers.js
index ab57e3d..28cbd32 100755
--- a/backend/jx_helpers.js
+++ b/backend/jx_helpers.js
@@ -28,11 +28,14 @@ exports.store = {
 };
 
 exports.addEvent = function (event, cb) {
-    if (!events[event]) {
-        events[event] = [];
+    if(typeof cb === "function") {
+        if (!events[event]) {
+            events[event] = [];
+        }
+        events[event].push(cb);
+    } else {
+        throw new TypeError("Callback is not a function.");
     }
-
-    events[event].push(cb);
 };
 
 exports.emitEvent = function (event, param1, param2, param3) {
diff --git a/backend/jxm.js b/backend/jxm.js
index 2fb8317..aa70917 100755
--- a/backend/jxm.js
+++ b/backend/jxm.js
@@ -392,7 +392,11 @@ exports.setEngine = function (app) {
  * @param listener {function} - Callback function which will be invoked upon emit of the event.
  */
 exports.on = function (event, listener) {
-    helpers.addEvent(event, listener);
+    if(typeof listener === "function") {
+        helpers.addEvent(event, listener);
+    } else {
+        throw new TypeError("Listener is not a function.");
+    }
 };