diff --git a/src/viewport_gui.cpp b/src/viewport_gui.cpp
--- a/src/viewport_gui.cpp
+++ b/src/viewport_gui.cpp
@@ -101,7 +101,7 @@ public:
 		this->DrawWidgets();
 	}
 
-	virtual void OnClick(Point pt, int widget)
+	virtual void OnClick(Point pt, int widget, int click_count)
 	{
 		switch (widget) {
 			case EVW_ZOOMIN: DoZoomInOutWindow(ZOOM_IN,  this); break;