diff --git a/configurations/values.py b/configurations/values.py
index f517b7b..467a0f9 100644
--- a/configurations/values.py
+++ b/configurations/values.py
@@ -266,7 +266,8 @@ class ValidationMixin(object):
         else:
             raise ValueError('Cannot use validator of '
                              '{0} ({1!r})'.format(self, self.validator))
-        self.to_python(self.default)
+        if self.default:
+            self.to_python(self.default)
 
     def to_python(self, value):
         try:
diff --git a/docs/patterns.rst b/docs/patterns.rst
index 7b199c3..a064fde 100644
--- a/docs/patterns.rst
+++ b/docs/patterns.rst
@@ -71,7 +71,7 @@ a ``Configuration`` class::
 
     from configurations import Configuration
 
-    class Prod(Configuration, FullPageCaching):
+    class Prod(FullPageCaching, Configuration):
         DEBUG = False
         # ...
 
diff --git a/tests/test_values.py b/tests/test_values.py
index 3a9fd9e..2c3d5ec 100644
--- a/tests/test_values.py
+++ b/tests/test_values.py
@@ -216,6 +216,14 @@ class ValueTests(TestCase):
         with env(DJANGO_TEST='httb://spam.eggs'):
             self.assertRaises(ValueError, value.setup, 'TEST')
 
+    def test_url_values_with_no_default(self):
+        value = URLValue()  # no default
+        with env(DJANGO_TEST='http://spam.eggs'):
+            self.assertEqual(value.setup('TEST'), 'http://spam.eggs')
+
+    def test_url_values_with_wrong_default(self):
+        self.assertRaises(ValueError, URLValue, 'httb://spam.eggs')
+
     def test_ip_values(self):
         value = IPValue('0.0.0.0')
         with env(DJANGO_TEST='127.0.0.1'):