Postman Pre-Request Script to create JWT for Google Service Account

I am trying to create a JWT to get an access token for a Google Service Account to use in a Postman Pre-Request script.

But I keep getting this error in the Postman Console:

Error:

And in the Postman Response window I get:

There was an error in evaluating the Pre-request Script:  undefined: undefined

I localized the error to be failing here:

var jwt = KJUR.jws.JWS.sign(null, header, claimSet, privateKey);

I'm importing it by using a request to http://kjur.github.io/jsrsasign/jsrsasign-latest-all-min.js and saving it to an environment variable called jsrsasign then rehydrating it below with eval.

Pre-Request Script:

var navigator = {};
var window = {};
eval(pm.environment.get("jsrsasign"));

var scope = pm.environment.get('scope');
var iss = pm.environment.get('iss');
var privateKey = pm.environment.get('privateKey');

const header = {"alg" : "RS256", "typ" : "JWT"};

const claimSet =
{
  "iss": iss,
  "scope": scope,
  "aud":"https://oauth2.googleapis.com/token",
  "exp":KJUR.jws.IntDate.get("now + 1hour").toString(),
  "iat": KJUR.jws.IntDate.get("now").toString()
}

console.log(`header: ${ JSON.stringify(header)}`);
console.log(`claim set: ${ JSON.stringify(claimSet) }`);

var jwt = KJUR.jws.JWS.sign(null, header, claimSet, privateKey);

pm.environment.set('jwt', jwt);

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