httpsProxyAgent is not defined - node.js

Question

I am integrating linkedin authentication with my node.js by passport-linkedin.
It works fine initially. I am redirected to login/signup page. User is entering their details but then when it is coming to authentication I get the error
InternalOAuthError: Failed to obtain access token
at Strategy.OAuth2Strategy._createOAuthError (C:\Users\SU40006113\Desktop\second\node_modules\passport-oauth2\lib\strategy.js:379:17)
at C:\Users\SU40006113\Desktop\second\node_modules\passport-oauth2\lib\strategy.js:166:45
at C:\Users\SU40006113\Desktop\second\node_modules\oauth\lib\oauth2.js:199:18
at ClientRequest.<anonymous> (C:\Users\SU40006113\Desktop\second\node_modules\oauth\lib\oauth2.js:170:5)
at emitOne (events.js:116:13)
at ClientRequest.emit (events.js:211:7)
at TLSSocket.socketErrorListener (_http_client.js:387:9)
at emitOne (events.js:116:13)
at TLSSocket.emit (events.js:211:7)
at emitErrorNT (internal/streams/destroy.js:64:8)
at _combinedTickCallback (internal/process/next_tick.js:138:11)
at process._tickCallback (internal/process/next_tick.js:180:9)
I checked on stack overflow and got to know that it is because of corporate proxy. I added
var HttpsProxyAgent = require('https-proxy-agent');
if (process.env['https_proxy']) {
httpsProxyAgent = new HttpsProxyAgent(process.env['https_proxy']);
}
and
options.agent = httpsProxyAgent;
in oauth2.js. But now I am getting this error
ReferenceError: httpsProxyAgent is not defined
at exports.OAuth2._request (C:\Users\SU40006113\Desktop\second\node_modules\oauth\lib\oauth2.js:126:19)
at exports.OAuth2.getOAuthAccessToken (C:\Users\SU40006113\Desktop\second\node_modules\oauth\lib\oauth2.js:197:8)
at loaded (C:\Users\SU40006113\Desktop\second\node_modules\passport-oauth2\lib\strategy.js:164:20)
at NullStore.verify (C:\Users\SU40006113\Desktop\second\node_modules\passport-oauth2\lib\state\null.js:9:3)
at Strategy.OAuth2Strategy.authenticate (C:\Users\SU40006113\Desktop\second\node_modules\passport-oauth2\lib\strategy.js:210:26)
at attempt (C:\Users\SU40006113\Desktop\second\node_modules\passport\lib\middleware\authenticate.js:361:16)
at authenticate (C:\Users\SU40006113\Desktop\second\node_modules\passport\lib\middleware\authenticate.js:362:7)
at Layer.handle [as handle_request] (C:\Users\SU40006113\Desktop\second\node_modules\express\lib\router\layer.js:95:5)
at next (C:\Users\SU40006113\Desktop\second\node_modules\express\lib\router\route.js:137:13)
at Route.dispatch (C:\Users\SU40006113\Desktop\second\node_modules\express\lib\router\route.js:112:3)
at Layer.handle [as handle_request] (C:\Users\SU40006113\Desktop\second\node_modules\express\lib\router\layer.js:95:5)
at C:\Users\SU40006113\Desktop\second\node_modules\express\lib\router\index.js:281:22
at Function.process_params (C:\Users\SU40006113\Desktop\second\node_modules\express\lib\router\index.js:335:12)
at next (C:\Users\SU40006113\Desktop\second\node_modules\express\lib\router\index.js:275:10)
at Function.handle (C:\Users\SU40006113\Desktop\second\node_modules\express\lib\router\index.js:174:3)
at router (C:\Users\SU40006113\Desktop\second\node_modules\express\lib\router\index.js:47:12)
I have installed https-proxy-agent.
What's causing the error?