diff --git a/src/osk_gui.cpp b/src/osk_gui.cpp
--- a/src/osk_gui.cpp
+++ b/src/osk_gui.cpp
@@ -243,7 +243,7 @@ static void AddKey(NWidgetHorizontal *ho
 		hor->Add(leaf);
 	}
 
-	*biggest_index = max(*biggest_index, widnum);
+	*biggest_index = std::max(*biggest_index, widnum);
 }
 
 /** Construct the top row keys (cancel, ok, backspace). */