diff --git a/django/db/models/fields/__init__.py b/django/db/models/fields/__init__.py
index 70a7024c26..42b727d755 100644
--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -750,6 +750,13 @@ class DateTimeField(DateField):
         if isinstance(value, datetime.date):
             value = datetime.datetime(value.year, value.month, value.day)
             if settings.USE_TZ:
+                # For backwards compatibility, interpret naive datetimes in
+                # local time. This won't work during DST change, but we can't
+                # do much about it, so we let the exceptions percolate up the
+                # call stack.
+                warnings.warn(u"DateTimeField received a naive datetime (%s)"
+                              u" while time zone support is active." % value,
+                              RuntimeWarning)
                 default_timezone = timezone.get_default_timezone()
                 value = timezone.make_aware(value, default_timezone)
             return value