Index: db/models/fields/__init__.py
===================================================================
--- db/models/fields/__init__.py	(revision 5620)
+++ db/models/fields/__init__.py	(working copy)
@@ -735,7 +735,7 @@
                 func(new_data[upload_field_name]["filename"], new_data[upload_field_name]["content"], save)
 
     def get_directory_name(self):
-        return os.path.normpath(datetime.datetime.now().strftime(self.upload_to))
+        return os.path.normpath(datetime.datetime.now().strftime(str(self.upload_to)))
 
     def get_filename(self, filename):
         from django.utils.text import get_valid_filename