diff --git a/src/autoreplace_gui.cpp b/src/autoreplace_gui.cpp
index f6d819d612..e1f4cf174e 100644
--- a/src/autoreplace_gui.cpp
+++ b/src/autoreplace_gui.cpp
@@ -368,7 +368,7 @@ public:
 
 			case WID_RV_START_REPLACE: {
 				Dimension d = GetStringBoundingBox(STR_REPLACE_VEHICLES_START);
-				maxdim(d, GetStringListBoundingBox(_start_replace_dropdown));
+				d = maxdim(d, GetStringListBoundingBox(_start_replace_dropdown));
 				d.width += padding.width;
 				d.height += padding.height;
 				size = maxdim(size, d);
diff --git a/src/core/geometry_func.hpp b/src/core/geometry_func.hpp
index 175a8a2ba7..58ebd316ab 100644
--- a/src/core/geometry_func.hpp
+++ b/src/core/geometry_func.hpp
@@ -12,7 +12,7 @@
 
 #include "geometry_type.hpp"
 
-Dimension maxdim(const Dimension &d1, const Dimension &d2);
+[[nodiscard]] Dimension maxdim(const Dimension &d1, const Dimension &d2);
 
 /**
  * Check if a rectangle is empty.
diff --git a/src/genworld_gui.cpp b/src/genworld_gui.cpp
index cd203da979..827223e932 100644
--- a/src/genworld_gui.cpp
+++ b/src/genworld_gui.cpp
@@ -660,7 +660,7 @@ struct GenerateLandscapeWindow : public Window {
 			default:
 				return;
 		}
-		maxdim(d, GetStringListBoundingBox(strs));
+		d = maxdim(d, GetStringListBoundingBox(strs));
 		d.width += padding.width;
 		d.height += padding.height;
 		size = maxdim(size, d);