diff --git a/test/test_persistence.py b/test/test_persistence.py
index 998b2e4..3feee9d 100644
--- a/test/test_persistence.py
+++ b/test/test_persistence.py
@@ -13,6 +13,7 @@ from .sample_data import TEST_DATA, TEST_CITY_1
 class DatabaseTestCase(unittest.TestCase):
 
     def setUp(self):
+        self.old_db_url = os.environ.get('DATABASE_URL')
         os.environ['DATABASE_URL'] = 'sqlite:///:memory:'
         self.db = connect('sqlite:///:memory:')
         self.tbl = self.db['weather']
@@ -21,7 +22,10 @@ class DatabaseTestCase(unittest.TestCase):
 
     def tearDown(self):
         # ensure env variable was unset
-        del os.environ['DATABASE_URL']
+        if self.old_db_url is None:
+            del os.environ['DATABASE_URL']
+        else:
+            os.environ['DATABASE_URL'] = self.old_db_url
 
     def test_valid_database_url(self):
         assert self.db.url, os.environ['DATABASE_URL']