diff --git a/masque/file/oasis.py b/masque/file/oasis.py
index b5d0cd8..67ea644 100644
--- a/masque/file/oasis.py
+++ b/masque/file/oasis.py
@@ -565,8 +565,9 @@ def _shapes_to_elements(
                 path_type = next((k for k, v in path_cap_map.items() if v == shape.cap), None)    # reverse lookup
                 if path_type is None:
                     raise PatternError(f'OASIS writer does not support path cap {shape.cap}')
-                extension_start = (path_type, shape.cap_extensions[0] if shape.cap_extensions is not None else None)
-                extension_end = (path_type, shape.cap_extensions[1] if shape.cap_extensions is not None else None)
+                extensions = None if shape.cap_extensions is None else rint_cast(shape.cap_extensions)
+                extension_start = (path_type, extensions[0] if extensions is not None else None)
+                extension_end = (path_type, extensions[1] if extensions is not None else None)
                 path = fatrec.Path(
                     layer = layer,
                     datatype = datatype,