diff --git a/config/example.json b/config/example.json
index e5ac3aaf7b..8487b5159d 100644
--- a/config/example.json
+++ b/config/example.json
@@ -12,6 +12,7 @@
     ],
     "externs": [
       "externs/bingmaps.js",
+      "externs/cartodb.js",
       "externs/bootstrap.js",
       "externs/closure-compiler.js",
       "externs/esrijson.js",
diff --git a/config/examples-all.json b/config/examples-all.json
index 40d452b4b3..3eee28d504 100644
--- a/config/examples-all.json
+++ b/config/examples-all.json
@@ -13,6 +13,7 @@
     "externs": [
       "externs/bingmaps.js",
       "externs/bootstrap.js",
+      "externs/cartodb.js",
       "externs/closure-compiler.js",
       "externs/esrijson.js",
       "externs/example.js",
diff --git a/config/ol.json b/config/ol.json
index 37039fac71..eb542af685 100644
--- a/config/ol.json
+++ b/config/ol.json
@@ -4,6 +4,7 @@
   "compile": {
     "externs": [
       "externs/bingmaps.js",
+      "externs/cartodb.js",
       "externs/closure-compiler.js",
       "externs/esrijson.js",
       "externs/geojson.js",
diff --git a/externs/cartodb.js b/externs/cartodb.js
new file mode 100644
index 0000000000..7d3c15e029
--- /dev/null
+++ b/externs/cartodb.js
@@ -0,0 +1,34 @@
+/**
+ * @externs
+ */
+
+
+
+/**
+ * @constructor
+ */
+var CartoDBLayerInfo = function() {};
+
+
+/**
+ * @type {string}
+ */
+CartoDBLayerInfo.prototype.layergroupid;
+
+
+/**
+ * @type {string}
+ */
+CartoDBLayerInfo.prototype.updated_at;
+
+
+/**
+ * @type {Object}
+ */
+CartoDBLayerInfo.prototype.metadata;
+
+
+/**
+ * @type {{http: string, https: string}}
+ */
+CartoDBLayerInfo.prototype.cdn_url;
diff --git a/src/ol/source/cartodb.js b/src/ol/source/cartodb.js
index 45f77f70fc..9ff67ac463 100644
--- a/src/ol/source/cartodb.js
+++ b/src/ol/source/cartodb.js
@@ -34,7 +34,7 @@ ol.source.CartoDB = function(options) {
   this.config_ = options.config || {};
 
   /**
-   * @type {Object.<string, Object>}
+   * @type {!Object.<string, CartoDBLayerInfo>}
    * @private
    */
   this.templateCache_ = {};
@@ -128,7 +128,7 @@ ol.source.CartoDB.prototype.handleInitResponse_ = function(paramHash, event) {
   if (client.status >= 200 && client.status < 300) {
     var response;
     try {
-      response = /** @type {Object} */(JSON.parse(client.responseText));
+      response = /** @type {CartoDBLayerInfo} */(JSON.parse(client.responseText));
     } catch (err) {
       this.setState(ol.source.State.ERROR);
       return;
@@ -152,12 +152,11 @@ ol.source.CartoDB.prototype.handleInitError_ = function(event) {
 
 /**
  * Apply the new tile urls returned by carto db
- * @param {Object} data Result of carto db call.
+ * @param {CartoDBLayerInfo} data Result of carto db call.
  * @private
  */
 ol.source.CartoDB.prototype.applyTemplate_ = function(data) {
-  var layerId = data['layergroupid'];
-  var tilesUrl = 'https://' + data['cdn_url']['https'] + '/' + this.account_ +
-      '/api/v1/map/' + layerId + '/{z}/{x}/{y}.png';
+  var tilesUrl = 'https://' + data.cdn_url.https + '/' + this.account_ +
+      '/api/v1/map/' + data.layergroupid + '/{z}/{x}/{y}.png';
   this.setUrl(tilesUrl);
 };