don't let you add SYNC as a shard (WAS: Failed to insert data into shard, which is a replica set)

I built up a system like this:
2 replica set shard servers
1. [shard11, shard12]
2. [shard21, shard22, arbiter2]
1 config server and 1 aggregator use the same machine

Then create one database and one collection, that shard was enabled.
After inserting some data via python driver, I got error. Please check attachment.