Index: django/db/backends/__init__.py
===================================================================
--- django/db/backends/__init__.py	(revision 17806)
+++ django/db/backends/__init__.py	(working copy)
@@ -568,7 +568,7 @@
         result set. If any fields are given, only the given fields are being
         checked for duplicates.
         """
-        if fields:
+        if not fields:
             raise NotImplementedError('DISTINCT ON fields is not supported by this database backend')
         else:
             return 'DISTINCT'