diff --git a/src/ol/layer/layergroup.js b/src/ol/layer/layergroup.js
index c685a6ec2c..7338704c3e 100644
--- a/src/ol/layer/layergroup.js
+++ b/src/ol/layer/layergroup.js
@@ -56,7 +56,7 @@ ol.layer.Group = function(opt_options) {
       ol.Object.getChangeEventType(ol.layer.GroupProperty.LAYERS),
       this.handleLayersChanged_, false, this);
 
-  if (goog.isDef(layers)) {
+  if (goog.isDefAndNotNull(layers)) {
     if (goog.isArray(layers)) {
       layers = new ol.Collection(goog.array.clone(layers));
     } else {
@@ -145,13 +145,13 @@ ol.layer.Group.prototype.handleLayersRemove_ = function(collectionEvent) {
 
 
 /**
- * @return {ol.Collection.<ol.layer.Base>|undefined} Collection of
+ * @return {!ol.Collection.<ol.layer.Base>} Collection of
  * {@link ol.layer.Layer layers} that are part of this group.
  * @observable
  * @api stable
  */
 ol.layer.Group.prototype.getLayers = function() {
-  return /** @type {ol.Collection.<ol.layer.Base>|undefined} */ (this.get(
+  return /** @type {!ol.Collection.<ol.layer.Base>} */ (this.get(
       ol.layer.GroupProperty.LAYERS));
 };
 goog.exportProperty(
@@ -161,7 +161,7 @@ goog.exportProperty(
 
 
 /**
- * @param {ol.Collection.<ol.layer.Base>|undefined} layers Collection of
+ * @param {!ol.Collection.<ol.layer.Base>} layers Collection of
  * {@link ol.layer.Layer layers} that are part of this group.
  * @observable
  * @api stable
diff --git a/src/ol/map.js b/src/ol/map.js
index 245d40991a..0f3dce3e9d 100644
--- a/src/ol/map.js
+++ b/src/ol/map.js
@@ -495,7 +495,6 @@ ol.Map.prototype.addInteraction = function(interaction) {
  */
 ol.Map.prototype.addLayer = function(layer) {
   var layers = this.getLayerGroup().getLayers();
-  goog.asserts.assert(goog.isDef(layers));
   layers.push(layer);
 };
 
@@ -703,16 +702,12 @@ goog.exportProperty(
 
 /**
  * Get the collection of layers associated with this map.
- * @return {ol.Collection.<ol.layer.Base>|undefined} Layers.
+ * @return {!ol.Collection.<ol.layer.Base>} Layers.
  * @api stable
  */
 ol.Map.prototype.getLayers = function() {
-  var layerGroup = this.getLayerGroup();
-  if (goog.isDef(layerGroup)) {
-    return layerGroup.getLayers();
-  } else {
-    return undefined;
-  }
+  var layers = this.getLayerGroup().getLayers();
+  return layers;
 };
 
 
@@ -1147,7 +1142,6 @@ ol.Map.prototype.removeInteraction = function(interaction) {
  */
 ol.Map.prototype.removeLayer = function(layer) {
   var layers = this.getLayerGroup().getLayers();
-  goog.asserts.assert(goog.isDef(layers));
   return layers.remove(layer);
 };