diff --git a/oslo_config/sphinxext.py b/oslo_config/sphinxext.py
index 133028aa..1de23d2a 100644
--- a/oslo_config/sphinxext.py
+++ b/oslo_config/sphinxext.py
@@ -206,6 +206,8 @@ def _format_option_help(app, namespaces, split_namespaces):
                 else:
                     group_name = group
                     group = None
+                if group_name is None:
+                    group_name = 'DEFAULT'
                 lines = _format_group(
                     app=app,
                     namespace=namespace,
@@ -228,6 +230,8 @@ def _format_option_help(app, namespaces, split_namespaces):
                 else:
                     group_name = group
                     group = None
+                if group_name is None:
+                    group_name = 'DEFAULT'
                 group_objs.setdefault(group_name, group)
                 by_section.setdefault(group_name, []).extend(group_opts)
         for group_name, group_opts in sorted(by_section.items()):
diff --git a/oslo_config/tests/test_sphinxext.py b/oslo_config/tests/test_sphinxext.py
index 03e7cbb2..2a201967 100644
--- a/oslo_config/tests/test_sphinxext.py
+++ b/oslo_config/tests/test_sphinxext.py
@@ -403,14 +403,14 @@ class FormatOptionHelpTest(base.BaseTestCase):
         _format_group.assert_any_call(
             app=None,
             namespace='namespace1',
-            group_name=None,
+            group_name='DEFAULT',
             group_obj=None,
             opt_list=['opt1'],
         )
         _format_group.assert_any_call(
             app=None,
             namespace='namespace2',
-            group_name=None,
+            group_name='DEFAULT',
             group_obj=None,
             opt_list=['opt2'],
         )
@@ -429,7 +429,7 @@ class FormatOptionHelpTest(base.BaseTestCase):
         _format_group.assert_called_once_with(
             app=None,
             namespace=None,
-            group_name=None,
+            group_name='DEFAULT',
             group_obj=None,
             opt_list=['opt1', 'opt2'],
         )