Sendgrid API: "The provided authorization grant is invalid, expired, or revoked"

Question

I've dug through past Stack posts and nothing seems to fix the issue. I'm using the generated API key (NOT the API key ID), I've allowed all permissions, and I even tried opening a second account and trying it from there. Here is the code:

// Create new mail object.
var mail = new helper.Mail();

// Set 'from' for email.
var from = new helper.Email("owner@site.com", "First Last");
mail.setFrom(from)

// Set 'subject' for email.
mail.setSubject("SchemeBeam Email Verification");

// Create a new personalization, which will allow us to define
// an individual to send this email too, as well as define
// substitutions to use so we can replace a string, such as
// '%hash'%, in the email template with the real hash for this
// user.
var personalization = new helper.Personalization()
var to = new helper.Email("test1@example.com", "Example User");
personalization.addTo(to);

// Set hash substituion.
//var substitution = new helper.Substitution("%hash%", md5Hash);
//personalization.addSubstitution(substitution);

// Let's send it.
var sg = require('sendgrid')(process.env.SENDGRID_API_KEY);
var request = sg.emptyRequest({
  method: 'POST',
  path: '/v3/mail/send',
  body: mail.toJSON(),
});

sg.API(request, function(error, response) {
  // Handle the response here.
  console.log(response.statusCode);
  console.log(response.body);
  console.log(response.headers);
});

And here is the error:

{ errors: 
   [ { message: 'The provided authorization grant is invalid, expired, or revoked',
       field: null,
       help: null } ] }
{ server: 'nginx',
  date: 'Wed, 16 Nov 2016 17:45:14 GMT',
  'content-type': 'application/json',
  'content-length': '116',
  connection: 'close',
  'x-frame-options': 'DENY' }