handleMapPostrender returns undefined

Without this we get the following warning from the compiler:

JSC_EXPORTED_FUNCTION_UNKNOWN_RETURN_TYPE. Unable to determine return type for exported function ol.control.Control.prototype.handleMapPostrender at ../src/ol/control/control.js line 79 : 51

I'm not sure why explicitely specifying the return type is required here.

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) {};