diff --git a/dj_database_url.py b/dj_database_url.py
index 9ea982a..39f7405 100644
--- a/dj_database_url.py
+++ b/dj_database_url.py
@@ -52,7 +52,7 @@ def config(
     conn_max_age=0,
     conn_health_checks=False,
     ssl_require=False,
-    test_options={},
+    test_options=None,
 ):
     """Returns configured DATABASE dictionary from DATABASE_URL."""
     s = os.environ.get(env, default)
diff --git a/test_dj_database_url.py b/test_dj_database_url.py
index 7ad601c..a10625a 100644
--- a/test_dj_database_url.py
+++ b/test_dj_database_url.py
@@ -1,5 +1,6 @@
 import os
 import unittest
+from unittest import mock
 
 import dj_database_url
 
@@ -530,6 +531,16 @@ class DatabaseTestSuite(unittest.TestCase):
         assert "CONN_MAX_AGE" not in url
         assert "CONN_HEALTH_CHECKS" not in url
 
+    @mock.patch.dict(
+        os.environ,
+        {"DATABASE_URL": "postgres://user:password@instance.amazonaws.com:5431/d8r8?"},
+    )
+    def test_persistent_connection_variables_config(self):
+        url = dj_database_url.config(conn_max_age=600, conn_health_checks=True)
+
+        assert url["CONN_MAX_AGE"] == 600
+        assert url["CONN_HEALTH_CHECKS"] is True
+
 
 if __name__ == "__main__":
     unittest.main()