diff --git a/src/ol/animation.js b/src/ol/animation.js
index c0f4986afd..430eb6fa79 100644
--- a/src/ol/animation.js
+++ b/src/ol/animation.js
@@ -17,7 +17,8 @@ goog.require('ol.easing');
 ol.animation.bounce = function(options) {
   var resolution = options.resolution;
   var start = options.start ? options.start : goog.now();
-  var duration = ol.isDef(options.duration) ? options.duration : 1000;
+  var duration = ol.isDef(options.duration) ?
+      /** @type {number} */ (options.duration) : 1000;
   var easing = options.easing ?
       options.easing : ol.easing.upAndDown;
   return (
@@ -55,7 +56,8 @@ ol.animation.pan = function(options) {
   var start = options.start ? options.start : goog.now();
   var sourceX = source[0];
   var sourceY = source[1];
-  var duration = ol.isDef(options.duration) ? options.duration : 1000;
+  var duration = ol.isDef(options.duration) ?
+      /** @type {number} */ (options.duration) : 1000;
   var easing = options.easing ?
       options.easing : ol.easing.inAndOut;
   return (
@@ -93,7 +95,8 @@ ol.animation.pan = function(options) {
 ol.animation.rotate = function(options) {
   var sourceRotation = options.rotation ? options.rotation : 0;
   var start = options.start ? options.start : goog.now();
-  var duration = ol.isDef(options.duration) ? options.duration : 1000;
+  var duration = ol.isDef(options.duration) ?
+      /** @type {number} */ (options.duration) : 1000;
   var easing = options.easing ?
       options.easing : ol.easing.inAndOut;
   var anchor = options.anchor ?
@@ -139,7 +142,8 @@ ol.animation.rotate = function(options) {
 ol.animation.zoom = function(options) {
   var sourceResolution = options.resolution;
   var start = options.start ? options.start : goog.now();
-  var duration = ol.isDef(options.duration) ? options.duration : 1000;
+  var duration = ol.isDef(options.duration) ?
+      /** @type {number} */ (options.duration) : 1000;
   var easing = options.easing ?
       options.easing : ol.easing.inAndOut;
   return (