diff --git a/src/substrait/extension_registry/registry.py b/src/substrait/extension_registry/registry.py
index 7eae5a0..2ba813f 100644
--- a/src/substrait/extension_registry/registry.py
+++ b/src/substrait/extension_registry/registry.py
@@ -2,6 +2,7 @@
 
 import re
 from collections import defaultdict
+from importlib.resources import as_file
 from importlib.resources import files as importlib_files
 from pathlib import Path
 from typing import Optional, Union
@@ -34,10 +35,25 @@ def __init__(self, load_default_extensions=True) -> None:
         # extension relation's output schema can be derived during inference.
         self._extension_relations: dict = {}
         if load_default_extensions:
-            for fpath in importlib_files("substrait_extensions.extensions").glob(  # type: ignore
-                "functions*.yaml"
-            ):
-                self.register_extension_yaml(fpath)
+            # NB: iterate + filter instead of ``.glob("functions*.yaml")``.
+            # ``importlib.resources.files`` returns a ``Traversable``, which is
+            # not guaranteed to be a filesystem path: when
+            # ``substrait_extensions.extensions`` resolves as a namespace
+            # package the reader returns a ``MultiplexedPath``, and that type
+            # implements ``iterdir``/``open``/``joinpath`` but not ``glob``
+            # (calling ``.glob`` raises ``AttributeError``). ``iterdir`` is part
+            # of the ``Traversable`` protocol and works across ``Path``,
+            # ``MultiplexedPath``, and zip-based readers alike.
+            #
+            # ``as_file`` turns each entry into a real filesystem path for the
+            # duration of the ``with`` block (a no-op for on-disk resources, an
+            # extraction to a temp file for zip-backed ones). Without it,
+            # ``register_extension_yaml`` -> ``Path(fname)`` would raise
+            # ``TypeError`` on a ``zipfile.Path``.
+            for fpath in importlib_files("substrait_extensions.extensions").iterdir():
+                if fpath.name.startswith("functions") and fpath.name.endswith(".yaml"):
+                    with as_file(fpath) as real_path:
+                        self.register_extension_yaml(real_path)
 
     def register_extension_yaml(
         self,
diff --git a/tests/extension_registry/test_multiplexed_path.py b/tests/extension_registry/test_multiplexed_path.py
new file mode 100644
index 0000000..4038ba7
--- /dev/null
+++ b/tests/extension_registry/test_multiplexed_path.py
@@ -0,0 +1,73 @@
+"""Regression tests for loading default extensions off a non-``Path`` resource.
+
+``importlib.resources.files`` returns a ``Traversable``, not necessarily a
+filesystem ``Path``. The registry must load the bundled extension YAMLs
+regardless of which concrete ``Traversable`` the resource reader hands back:
+
+* ``MultiplexedPath`` -- returned for a *namespace* package (no ``__init__.py``).
+  It implements ``iterdir``/``open``/``joinpath`` but *not* ``glob``, so the old
+  ``files(...).glob("functions*.yaml")`` raised
+  ``AttributeError: 'MultiplexedPath' object has no attribute 'glob'`` (observed
+  under the pure-Python WASI guest interpreter).
+* ``zipfile.Path`` -- returned for a zip-imported package. It is not an
+  ``os.PathLike``, so ``register_extension_yaml``'s ``Path(fname)`` raised
+  ``TypeError``; ``as_file`` now materialises a real path first.
+"""
+
+import zipfile
+from importlib.resources import files
+
+import substrait.extension_registry.registry as registry_module
+from substrait.builders.type import i8
+from substrait.extension_registry import ExtensionRegistry
+
+try:  # Python 3.11+
+    from importlib.resources.readers import MultiplexedPath
+except ModuleNotFoundError:  # Python 3.10
+    from importlib.readers import MultiplexedPath
+
+
+def _assert_defaults_loaded(reg: ExtensionRegistry) -> None:
+    """The default extension set was parsed and registered."""
+    assert reg._function_mapping
+    assert reg.lookup_function(
+        urn="extension:io.substrait:functions_arithmetic",
+        function_name="add",
+        signature=[i8(nullable=False), i8(nullable=False)],
+    )
+
+
+def test_load_default_extensions_via_multiplexed_path(monkeypatch):
+    # Wrap the real extensions directory in a MultiplexedPath so ``files()``
+    # yields exactly what it would for a namespace package. Guard the premise:
+    # MultiplexedPath must not expose ``glob`` (otherwise this test proves
+    # nothing).
+    ext_dir = files("substrait_extensions.extensions")
+    multiplexed = MultiplexedPath(ext_dir)
+    assert not hasattr(multiplexed, "glob")
+
+    monkeypatch.setattr(registry_module, "importlib_files", lambda _pkg: multiplexed)
+
+    # Previously raised AttributeError here; must now load cleanly.
+    reg = ExtensionRegistry(load_default_extensions=True)
+    _assert_defaults_loaded(reg)
+
+
+def test_load_default_extensions_via_zipfile_path(monkeypatch, tmp_path):
+    # Mirror a zip-imported install: copy the real YAMLs into a zip and resolve
+    # ``files()`` to a zipfile.Path over the archive. The entries are not
+    # os.PathLike, so register_extension_yaml's ``Path(fname)`` would raise
+    # TypeError without the ``as_file`` materialisation.
+    ext_dir = files("substrait_extensions.extensions")
+    archive = tmp_path / "substrait_extensions.zip"
+    with zipfile.ZipFile(archive, "w") as zf:
+        for child in ext_dir.iterdir():
+            if child.name.endswith(".yaml"):
+                zf.writestr(f"extensions/{child.name}", child.read_bytes())
+
+    zip_dir = zipfile.Path(archive, "extensions/")
+    monkeypatch.setattr(registry_module, "importlib_files", lambda _pkg: zip_dir)
+
+    # Previously raised TypeError in register_extension_yaml; must now load.
+    reg = ExtensionRegistry(load_default_extensions=True)
+    _assert_defaults_loaded(reg)