diff --git a/src/gui.h b/src/gui.h
index dd20d293fb..cfd333cb45 100644
--- a/src/gui.h
+++ b/src/gui.h
@@ -56,7 +56,7 @@ void ShowEstimatedCostOrIncome(Money cost, int x, int y);
 
 void ShowExtraViewportWindow(TileIndex tile = INVALID_TILE);
 namespace citymania {
-Window *ShowExtraViewportWindow(int x, int y, int z);
+void ShowExtraViewportWindow(int x, int y, int z);
 }
 void ShowExtraViewportWindowForTileUnderCursor();