When trying to add a replica set as a new shard and specifying an arbiter among hosts, it replies with error like "host XXX does not belong to replica set YYY" (when it really does).

During discussion at https://groups.google.com/forum/?hl=en&pli=1#!topic/mongodb-user/raZ8mJteML8 it turned out that currently you can't add arbiter to a shard. And, indeed, if you omit it from the command, everything works.

Also, when you specify non-existent host, the error message is the same!

RESOLUTION: provide two distinctive messages for two cases described above.

• host XXX is an arbiter. Can't add arbiter to shard.
• host XXX is unreachable. Check your hostname/port.