lab throws with ReferenceError: __$$labCov is not defined when run with coverage enabled and no code to cover

[nlf]

To reproduce:

mkdir lab-test
cd lab-test
npm i lab
mkdir test

Create the file test/index.js with the following contents:

var Lab = require('lab');

var lab = exports.lab = Lab.script();
var expect = Lab.expect;
var describe = lab.experiment;
var it = lab.test;


describe('math', function () {

    it('adds one and one', function (done) {

        expect(1+1).to.equal(2);
        done();
    });
});

Run the tests with coverage enabled: ./node_modules/.bin/lab -t 100

Obviously, this is a contrived example, but I noticed it when I was creating a template for an empty test suite and thought I should bring it up since crashes are rarely ever the desired result of anything.