ember-cli-mirage error: Nothing returned by handler, but handler exists

In the app I'm working with, we have a GET route that validates a user's email address. If the email is invalid, the server responds with:

• a 200 status code
• response headers with Content-Type:application/json; charset=utf-8
• and the response data itself is just a string of "This email is invalid"

I'm trying to simulate this in ember-cli-mirage by doing:

this.get('/ember_api/v1/validations/validate_email', function() {
  return [200, { 'Content-Type': 'application/json' }, "This email is invalid"];

  // also tried this:
  // return new Mirage.Response(200, { 'Content-Type': 'application/json' }, JSON.stringify({"message":"This email is invalid"}));

  // and tried this:
  // return "This email is invalid";
});

The test itself is a button click that fires off this request:

GET "/ember_api/v1/validations/validate_email?email=fakey%40fakefakefake.com&skip_uniq=true"

and the error I'm getting is:

Pretender intercepted GET /ember_api/v1/validations/validate_email?email=tom%40gmail.com&skip_uniq=true but encountered an error: Nothing returned by handler for /ember_api/v1/validations/validate_email?email=tom%40gmail.com&skip_uniq=true. Remember to return [status, headers, body]; in your route handler.

It's asking me to return [status, headers, body], but I'm doing this in my handler and it still throws the error. Is this actually an issue with the response? Do I need to edit my API to actually return a JSONAPI formatted object so I can write the test that way? I feel like I should be able to return a string in my test since that's what the app is doing. Any help is appreciated.