diff --git a/externs/oli.js b/externs/oli.js
index f6feed46c6..ef12b7092d 100644
--- a/externs/oli.js
+++ b/externs/oli.js
@@ -17,6 +17,7 @@ oli.control.Control = function() {};
 
 /**
  * @param {ol.MapEvent} mapEvent Map event.
+ * @return {undefined} Undefined.
  */
 oli.control.Control.prototype.handleMapPostrender = function(mapEvent) {};