diff --git a/src/ol/sphere.js b/src/ol/sphere.js
index 7734d8954c..ba215aac57 100644
--- a/src/ol/sphere.js
+++ b/src/ol/sphere.js
@@ -106,7 +106,7 @@ ol.Sphere.DEFAULT_RADIUS = 6371008.8;
  * the sum of all rings.  For points, the length is zero.  For multi-part
  * geometries, the length is the sum of the length of each part.
  * @param {ol.geom.Geometry} geometry A geometry.
- * @param {olx.SphereMetricOptions} opt_options Options for the length
+ * @param {olx.SphereMetricOptions=} opt_options Options for the length
  *     calculation.  By default, geometries are assumed to be in 'EPSG:3857'.
  *     You can change this by providing a `projection` option.
  * @return {number} The spherical length (in meters).
@@ -201,7 +201,7 @@ ol.Sphere.getDistance_ = function(c1, c2, radius) {
  * Get the spherical area of a geometry.  This is the area (in meters) assuming
  * that polygon edges are segments of great circles on a sphere.
  * @param {ol.geom.Geometry} geometry A geometry.
- * @param {olx.SphereMetricOptions} opt_options Options for the area
+ * @param {olx.SphereMetricOptions=} opt_options Options for the area
  *     calculation.  By default, geometries are assumed to be in 'EPSG:3857'.
  *     You can change this by providing a `projection` option.
  * @return {number} The spherical area (in square meters).