Typo fix for an error path in r100090.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10091 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/query.py

@@ -609,7 +609,7 @@ def only(self, *fields):
         method and that are not already specified as deferred are loaded
         immediately when the queryset is evaluated.
         """
-        if fields == [None]:
+        if fields == (None,):
             # Can only pass None to defer(), not only(), as the rest option.
             # That won't stop people trying to do this, so let's be explicit.
             raise TypeError("Cannot pass None as an argument to only().")