--- a/migration.py	Wed Jun 29 16:02:24 2011 +0200
+++ b/migration.py	Wed Jun 29 14:05:14 2011 +0200
@@ -509,6 +509,8 @@
                         if version_strictly_lower(version, ver):
                             version = ver
                             source = src
+                    elif op == None:
+                        continue
                     else:
                         print 'unable to handle this case', oper, version, op, ver
             # "solve" constraint satisfaction problem