/srv/node_modules/mongodb-core/lib/topologies/replset_state.js:149
  var serverName = server.name.toLowerCase();
                         ^

TypeError: Cannot read property 'name' of null
    at ReplSetState.remove (/srv/node_modules/mongodb-core/lib/topologies/replset_state.js:149:26)
    at Object.handler [as cb] (/srv/node_modules/mongodb-core/lib/topologies/replset.js:1221:28)
    at Connection.<anonymous> (/srv/node_modules/mongodb-core/lib/connection/pool.js:299:33)
    at Connection.g (events.js:291:16)
    at emitTwo (events.js:106:13)
    at Connection.emit (events.js:191:7)
    at Socket.<anonymous> (/srv/node_modules/mongodb-core/lib/connection/connection.js:256:10)
    at Socket.g (events.js:291:16)
    at emitNone (events.js:86:13)
    at Socket.emit (events.js:185:7)