diff --git a/src/ol/style.js b/src/ol/style.js
index 97ed81deb3..87e1b86e6e 100644
--- a/src/ol/style.js
+++ b/src/ol/style.js
@@ -30,12 +30,13 @@ ol.style.Stroke;
 
 
 /**
- * @param {ol.style.Stroke} strokeStyle1 Stroke style 1.
- * @param {ol.style.Stroke} strokeStyle2 Stroke style 2.
+ * @param {?ol.style.Stroke} strokeStyle1 Stroke style 1.
+ * @param {?ol.style.Stroke} strokeStyle2 Stroke style 2.
  * @return {boolean} Equals.
  */
 ol.style.stroke.equals = function(strokeStyle1, strokeStyle2) {
   return strokeStyle1 === strokeStyle2 || (
+      !goog.isNull(strokeStyle1) && !goog.isNull(strokeStyle2) &&
       strokeStyle1.color == strokeStyle2.color &&
       strokeStyle1.width == strokeStyle2.width);
 };