Problem while using merge join

Hi,

I try to use a merge join for 2 bags. Here is my pig code:
http://pastebin.com/Y9b2UtNk .

But I got this error:

Caused by:
org.apache.pig.backend.hadoop.executionengine.physicalLayer.LogicalToPhysicalTranslatorException:
ERROR 1103: Merge join/Cogroup only supports Filter, Foreach, Ascending
Sort, or Load as its predecessors. Found

I think the reason is that there is no sort function or something like
this. But the bags are definitely sorted. How can I do the merge join?

thanks