MongoNetworkError: [Error: unable to get local issuer certificate

I have my mongo details in Apache config file. And getting this error.

SetEnv MONGO_URL mongodb://:@XXXXXXXXX.docdb.amazonaws.com:27017/?ssl=true&ssl_ca_certs=/home/ec2-user/rds-combined-ca-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false

App 2291 output: MongoNetworkError: failed to connect to server [XXXXXX.docdb.amazonaws.com:27017] on first connect [Error: unable to get local issuer certificate
App 2291 output: at TLSSocket.onConnectSecure (_tls_wrap.js:1474:34)
App 2291 output: at TLSSocket.emit (events.js:310:20)
App 2291 output: at TLSSocket.EventEmitter.emit (domain.js:482:12)
App 2291 output: at TLSSocket._finishInit (_tls_wrap.js:917:8)
App 2291 output: at TLSWrap.ssl.onhandshakedone (_tls_wrap.js:687:12) {
App 2291 output: name: ‘MongoNetworkError’,
App 2291 output: [Symbol(mongoErrorContextSymbol)]: {}
App 2291 output: }]