diff --git a/tests/test_domain_py.py b/tests/test_domain_py.py
index 947e90d34..178fe18cf 100644
--- a/tests/test_domain_py.py
+++ b/tests/test_domain_py.py
@@ -626,3 +626,26 @@ def test_module_index_not_collapsed(app):
          ('s', [IndexEntry('sphinx', 0, 'index', 'module-sphinx', '', '', '')])],
         True
     )
+
+
+@pytest.mark.sphinx(freshenv=True, confoverrides={'modindex_common_prefix': ['sphinx.']})
+def test_modindex_common_prefix(app):
+    text = (".. py:module:: docutils\n"
+            ".. py:module:: sphinx\n"
+            ".. py:module:: sphinx.config\n"
+            ".. py:module:: sphinx.builders\n"
+            ".. py:module:: sphinx.builders.html\n"
+            ".. py:module:: sphinx_intl\n")
+    restructuredtext.parse(app, text)
+    index = PythonModuleIndex(app.env.get_domain('py'))
+    assert index.generate() == (
+        [('b', [IndexEntry('sphinx.builders', 1, 'index', 'module-sphinx.builders', '', '', ''),  # NOQA
+                IndexEntry('sphinx.builders.html', 2, 'index', 'module-sphinx.builders.html', '', '', '')]),  # NOQA
+         ('c', [IndexEntry('sphinx.config', 0, 'index', 'module-sphinx.config', '', '', '')]),
+         ('d', [IndexEntry('docutils', 0, 'index', 'module-docutils', '', '', '')]),
+         ('s', [IndexEntry('sphinx', 0, 'index', 'module-sphinx', '', '', ''),
+                IndexEntry('sphinx_intl', 0, 'index', 'module-sphinx_intl', '', '', '')])],
+        True
+    )
+
+