Node error on Heroku instance: Unable to verify first certificate

I am trying to run an API provided via heroku. Currently I'm using SSL encryption and I've added it to the site via the panel domains and certificatesin the Settings page . While I can connect to the site on the browser using https and postman, I can't access the API using the node.js instance, I get the following error:

{ Error: unable to verify the first certificate
   at TLSSocket.<anonymous> (_tls_wrap.js:1062:38)
   at emitNone (events.js:86:13)
   at TLSSocket.emit (events.js:185:7)
   at TLSSocket._finishInit (_tls_wrap.js:586:8)
at TLSWrap.ssl.onhandshakedone (_tls_wrap.js:416:38) code: 
'UNABLE_TO_VERIFY_LEAF_SIGNATURE' }

I've tried using the ssl-root-casmodule, but it doesn't seem to work on the heroku instance as it requires downloading the intermeidate certificate and restarting the server. However, restarting the heroku instance ssl-root-casonly loses the downloaded certificate. I also can't provide the SSL certificate directly from my node server. I need help with this issue, which I believe is the result of improper bundling of intermediate certificates.