When a replset contains a default write concern via getLastErrorDefaults, if the set is reconfigured to then remove this, explicit calls to getLastError will fail with:

 {
        "lastOp" : Timestamp(1431320964, 3),
        "connectionId" : 1,
        "n" : 0,
        "badGLE" : {

        },
        "ok" : 0,
        "errmsg" : "write concern object cannot be empty",
        "code" : 9
}

Write commands are unaffected, and work as expected.

Effective workarounds are:

1. Restarting the affected mongod(s).
2. Re-adding getLastErrorDefaults as { w: 1 }

Stepping down the primary is NOT a workaround (the new primary will have the same problem).

Reproducer jstest attached.

This doesn't affect 2.4.x (reason unknown), or 3.0.x/3.1.x (because in those versions, replset configs always explicitly include all options).