diff --git a/importlib_resources/_common.py b/importlib_resources/_common.py
index 3e665b9..4e7c913 100644
--- a/importlib_resources/_common.py
+++ b/importlib_resources/_common.py
@@ -71,6 +71,19 @@ def is_wrapper(frame_info):
     return next(callers).frame
 
 
+def _assert_spec(package: types.ModuleType) -> None:
+    """
+    Provide a nicer error message when package is ``__main__``
+    and its ``__spec__`` is ``None``
+    (https://docs.python.org/3/reference/import.html#main-spec).
+    """
+    if package.__spec__ is None:
+        raise TypeError(
+            f"Cannot access resources for '{package.__name__}' "
+            "as it does not appear to correspond to an importable module (its __spec__ is None)."
+        )
+
+
 def from_package(package: types.ModuleType):
     """
     Return a Traversable object for the given package.
@@ -79,6 +92,7 @@ def from_package(package: types.ModuleType):
     # deferred for performance (python/cpython#109829)
     from .future.adapters import wrap_spec
 
+    _assert_spec(package)
     spec = wrap_spec(package)
     reader = spec.loader.get_resource_reader(spec.name)
     return reader.files()
diff --git a/importlib_resources/tests/test_resource.py b/importlib_resources/tests/test_resource.py
index ed9fa19..63a8bfb 100644
--- a/importlib_resources/tests/test_resource.py
+++ b/importlib_resources/tests/test_resource.py
@@ -1,3 +1,4 @@
+import types
 import unittest
 from importlib import import_module
 
@@ -219,5 +220,24 @@ class ResourceFromNamespaceZipTests(
     MODULE = 'namespacedata01'
 
 
+class MainModuleTests(unittest.TestCase):
+    def test_main_module_with_none_spec(self):
+        """
+        __main__ module with no spec should raise TypeError (for clarity).
+
+        See python/cpython#138531 for details.
+        """
+        # construct a __main__ module with no __spec__.
+        mainmodule = types.ModuleType("__main__")
+
+        assert mainmodule.__spec__ is None
+
+        with self.assertRaises(
+            TypeError,
+            msg="Cannot access resources for '__main__' as it does not appear to correspond to an importable module (its __spec__ is None).",
+        ):
+            resources.files(mainmodule)
+
+
 if __name__ == '__main__':
     unittest.main()
diff --git a/newsfragments/331.feature.rst b/newsfragments/331.feature.rst
new file mode 100644
index 0000000..04e9344
--- /dev/null
+++ b/newsfragments/331.feature.rst
@@ -0,0 +1 @@
+``files()`` now provides a nicer error when __main__.__spec__ is None.