diff --git a/pelican/readers.py b/pelican/readers.py
index 811b5b6e..ab329f6e 100644
--- a/pelican/readers.py
+++ b/pelican/readers.py
@@ -532,7 +532,7 @@ def default_metadata(settings=None, process=None):
                 value = process('category', value)
             metadata['category'] = value
         if settings.get('DEFAULT_DATE', None) and settings['DEFAULT_DATE'] != 'fs':
-            if isinstance(settings['DEFAULT_DATE'], basestring):
+            if isinstance(settings['DEFAULT_DATE'], six.string_types):
                 metadata['date'] = get_date(settings['DEFAULT_DATE'])
             else:
                 metadata['date'] = SafeDatetime(*settings['DEFAULT_DATE'])