MongoDB: Server sockets closed after a few minutes

I am working with multiple AWS intances connected to the same mongo database (inside Compose.io Elastic deployment) but I keep getting the error server <url>:<port> sockets closed after a few minutes. Can anyone give me any hint about what may be wrong with the connection code?

CONNECTION CODE

var url = "http://mongodb<user>:<password>@<url1>:<port1>,<url2>:<port2>/<dbName>?replicaSet=<replicaSetName>"; 
var options = { 
    server : {"socketOptions.keepAlive": 1}, 
    replSet : { "replicaSet": <replicaSetName>, "socketOptions.keepAlive": 1 } 
}; 
MongoClient.connect(url, options, function(err, db) { ... });

ERROR MESSAGE

Potentially unhandled rejection [2] MongoError: server <url>:<port> sockets closed 
at null. (/var/app/current/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:328:47) 
at g (events.js:199:16) 
at emit (events.js:110:17) 
at null. (/var/app/current/node_modules/mongodb/node_modules/mongodb-core/lib/connection/pool.js:101:12) 
at g (events.js:199:16) 
at emit (events.js:110:17) 
at Socket. (/var/app/current/node_modules/mongodb/node_modules/mongodb-core/lib/connection/connection.js:142:12) 
at Socket.g (events.js:199:16) 
at Socket.emit (events.js:107:17) 
at TCP.close (net.js:485:12)