diff --git a/src/ol/source/vectorsource.js b/src/ol/source/vectorsource.js
index f17d5ebd74..251f66ba38 100644
--- a/src/ol/source/vectorsource.js
+++ b/src/ol/source/vectorsource.js
@@ -659,7 +659,7 @@ ol.source.Vector.prototype.getClosestFeatureToCoordinate =
  *
  * This method is not available when the source is configured with
  * `useSpatialIndex` set to `false`.
- * @return {ol.Extent} Extent.
+ * @return {!ol.Extent} Extent.
  * @api stable
  */
 ol.source.Vector.prototype.getExtent = function() {
diff --git a/src/ol/structs/rbush.js b/src/ol/structs/rbush.js
index deef53d687..41c6d2cd20 100644
--- a/src/ol/structs/rbush.js
+++ b/src/ol/structs/rbush.js
@@ -253,7 +253,7 @@ ol.structs.RBush.prototype.clear = function() {
 
 /**
  * @param {ol.Extent=} opt_extent Extent.
- * @return {ol.Extent} Extent.
+ * @return {!ol.Extent} Extent.
  */
 ol.structs.RBush.prototype.getExtent = function(opt_extent) {
   // FIXME add getExtent() to rbush