When using rs.initiate with a configuration object that contains an arbiter which has had a tag, configuration is saved successfully and replica set comes online. When reconfiguring using rs.reconfig the following error is generated:

{
"assertion" : "arbiters cannot have tags",
"assertionCode" : 14827,
"errmsg" : "db assertion failure",
"ok" : 0
}

The behaviour is inconsistent.

Here is an example configuration session (note host / shard names changed):

> cfg = {
"_id" : "shard-A",
"members" : [
{
"_id" : 0,
"host" : "host-1",
"priority" : 4,
"tags" :

{ "dc" : "east" }

,
"votes" : 1
},
{
"_id" : 1,
"host" : "host-2",
"priority" : 3,
"tags" :

{ "dc" : "east" }

,
"votes" : 1
},
{
"_id" : 2,
"host" : "host-3",
"priority" : 1,
"tags" :

{ "dc" : "west" }

,
"votes" : 1
},
{
"_id" : 3,
"host" : "host-4,
"priority" : 2,
"tags" :

{ "dc" : "west" }

,
"votes" : 1
},
{
"_id" : 99,
"host" : "host-5",
"tags" :

{ "dc" : "west" }

,
"votes" : 1,
"arbiterOnly" : true
}
]
};

> rs.initiate(cfg);
{
"info" : "Config now saved locally. Should come online in about a minute.",
"ok" : 1
}

> rs.reconfig(cfg,

{ force: true}

);
{
"assertion" : "arbiters cannot have tags",
"assertionCode" : 14827,
"errmsg" : "db assertion failure",
"ok" : 0
}