--- a/roundup/backends/rdbms_common.py	Mon Jul 08 11:14:53 2013 +0200
+++ b/roundup/backends/rdbms_common.py	Mon Jul 08 16:55:20 2013 +0200
@@ -2419,7 +2419,7 @@
                         # where clause will always be false, and we
                         # can optimize the query away.
                         if not v:
-                            return []
+                            return None
                         s = ','.join([a for x in v])
                         where.append('_%s.%s in (%s)'%(pln, k, s))
                         args = args + v