diff --git a/tests/test_extension.py b/tests/test_extension.py
index e2e92903..baf77fbf 100644
--- a/tests/test_extension.py
+++ b/tests/test_extension.py
@@ -7,11 +7,19 @@
 from markdown import Markdown
 from mkdocs import config
 
+try:
+    from mkdocs.config.defaults import get_schema
+except ImportError:
+
+    def get_schema():
+        """Fallback for old versions of MkDocs."""
+        return config.DEFAULT_SCHEMA
+
 
 @pytest.fixture(name="ext_markdown")
 def fixture_ext_markdown(request, tmp_path):
     """Yield a Markdown instance with MkdocstringsExtension, with config adjustments."""
-    conf = config.Config(schema=config.DEFAULT_SCHEMA)
+    conf = config.Config(schema=get_schema())
 
     conf_dict = {
         "site_name": "foo",