diff --git a/masque/file/gdsii.py b/masque/file/gdsii.py
index fa49eb3..c20200c 100644
--- a/masque/file/gdsii.py
+++ b/masque/file/gdsii.py
@@ -453,7 +453,7 @@ def _shapes_to_elements(
 
                 extension: tuple[int, int]
                 if shape.cap == Path.Cap.SquareCustom and shape.cap_extensions is not None:
-                    extension = tuple(shape.cap_extensions)     # type: ignore
+                    extension = tuple(rint_cast(shape.cap_extensions))
                 else:
                     extension = (0, 0)