mongos> use bug
switched to db bug
mongos> db.t.insert(

{'_id':[1,2,3]}

)
_id cannot be an array

mongos> use admin
switched to db admin
mongos> db.runCommand(

{ enablesharding : "bug" }

);

{ "ok" : 1 }

mongos> db.runCommand( { shardcollection : "bug.s", key :

{ _id : 1 }

} )

{ "collectionsharded" : "bug.s", "ok" : 1 }

mongos> use bug
switched to db bug
mongos> db.s.insert(

{'_id':[1,2,3]}

)
mongos> db.s.find()

{ "_id" : ObjectId("4feb209a465c15a3483518f7"), "_id" : ObjectId("4feb209a465c15a3483518f7") }