MongooseServerSelectionError: self signed certificate in certificate chain

Why am I getting this error: MongooseServerSelectionError: self signed certificate in certificate chain.

I’m using mongoose (v6.8.3) to connect to my mongodb database. As far as I know my code should work just fine:

let options = {
  useNewUrlParser: true,
  useUnifiedTopology: true,
};
const dbCertPath = path.resolve('./ca-certificate.crt');

if (envVars.CA_CERT) {
  fs.writeFileSync(dbCertPath, envVars.CA_CERT);
  options = {
    useNewUrlParser: true,
    useUnifiedTopology: true,
    ssl: true,
    sslCA: `${dbCertPath}`,
  };
}

mongoose
  .connect(mongodbUrl, options)
  .then(() => {
    logger.info('Connected to MongoDB');
  })
  .catch((error) => {
    logger.error(error);
  });