Box Node SDK TypeError when refreshing App Auth AccessToken on heroku.

Hi,

I am having trouble authenticating to Box with the Node Box SDK when deployed on heroku.

This is the error I am getting: 

TypeError: key must be a string or buffer
 at typeError (/app/node_modules/jwa/index.js:14:10)
 at Object.sign (/app/node_modules/jwa/index.js:48:13)
 at Object.jwsSign [as sign] (/app/node_modules/jws/lib/sign-stream.js:23:26)
 at Object.JWT.sign (/app/node_modules/jsonwebtoken/index.js:137:16)
 at TokenManager.getTokensJWTGrant (/app/node_modules/box-node-sdk/lib/token-manager.js:329:24)
 at AppAuthSession._refreshAppAuthAccessToken (/app/node_modules/box-node-sdk/lib/sessions/app-auth-session.js:77:20)
 at AppAuthSession.getAccessToken (/app/node_modules/box-node-sdk/lib/sessions/app-auth-session.js:110:8)
 at BoxClient.upload (/app/node_modules/box-node-sdk/lib/box-client.js:522:7)
 at BoxClient._makeRequest (/app/node_modules/box-node-sdk/lib/box-client.js:208:16)
 at Files.uploadFile (/app/node_modules/box-node-sdk/lib/managers/files.js:540:14)
 .....

This happens when trying to interact with the Box API using the SDK. For example with the following line of code: 

client._session._refreshAppAuthAccessToken((err, accessToken) => { ...

But also with uploading: 

client.files.uploadFile('0', fileToUpload.filename, stream, (err, upload) => { ...

This error only exists when the application I built is deployed to heroku and not on my development environment. The key works perfectlly fine on the development environment.

This is de code I am using to initialise the Box SDK: 

const boxSDK = new BoxSDK({
  clientID: 'MY_CLIENT_ID',
  clientSecret: 'MY_CLIENT_SECRET',
  appAuth: {
    keyID: 'MY_KEY_ID',
    privateKey: 'MY_PRIVATE_KEY',
    passphrase: 'MY_PASSPHRASE'
  }
});

const client = boxSDK.getAppAuthClient('enterprise', 'MY_ENTERPRISE_ID');

I have tried using different versions of node (4.7.3, 6.1.0, 7.5.0). All work on the development environment but not when deployed to heroku.

I have tried logging the privateKey variable before interaction with the Box API and the variable has the correct value.

Did I miss something?