diff --git a/src/smallmap_gui.cpp b/src/smallmap_gui.cpp
--- a/src/smallmap_gui.cpp
+++ b/src/smallmap_gui.cpp
@@ -1423,12 +1423,15 @@ public:
 	}
 
 	/**
-	 * Notifications for the smallmap window.
+	 * Some data on this window has become invalid.
+	 * @param data Information about the changed data.
 	 * - data = 0: Displayed industries at the industry chain window have changed.
 	 * - data = 1: Companies have changed.
+	 * @param gui_scope Whether the call is done from GUI scope. You may not do everything when not in GUI scope. See #InvalidateWindowData() for details.
 	 */
-	virtual void OnInvalidateData(int data)
+	virtual void OnInvalidateData(int data = 0, bool gui_scope = true)
 	{
+		if (!gui_scope) return;
 		switch (data) {
 			case 1:
 				/* The owner legend has already been rebuilt. */