Travis CI build process - TypeScript catching an error before Mocha does

This isn't so much a bug as the TypeScript compiler doing its job, however it is causing my Travis builds to fail.

In my package I have a function completeRound which takes a number as its first argument, then 3 optional arguments, and because I've written it in TypeScript I tell it that it needs to have a number passed in as the first argument:

function completeRound( number: number, rounding = 1, direction = 'closest', offset = 0 ): number {
  let n: number = +number,
      r: number = Math.abs(+rounding),
      d: string = direction,
      o: number = +offset;

  if (typeof n !== 'number') {
    throw new TypeError('You need to round a number!');
  }

  // Rest of the code goes here
}

It all works fine, the only trouble is when it comes to testing. In my testing script I have:

import completeRound from '../src/completeRound';
import { expect } from 'chai';
import 'mocha';

const expectedResults = [
  {
    testVal : {
      number    : 3.5,
      rounding  : 1,
      direction : 'down'
    },
    eR      : 3
  },
  // 40-something different tests, all passing fine
];

expectedResults.map(t => describe(`completeRound(${t.testVal.number}, ${t.testVal.rounding}, '${t.testVal.direction}', ${t.testVal.offset})`,() => {
  it(`should return ${t.eR}`, () => {
    const result = completeRound(t.testVal.number,t.testVal.rounding,t.testVal.direction,t.testVal.offset);
    expect(result).to.equal(t.eR);
  });
})); // For testing results where it shouldn't throw an error

/* This one is the problem line */
expect(() => completeRound([5])).to.throw(TypeError, /a number/);
/* ---------------------------- */

expect(() => completeRound(5,1,'arriba')).to.throw(Error, /valid rounding direction/);

So I'm passing it an array instead of a number, where I want JavaScript to throw the TypeError. Obviously I want people using this either in JavaScript or in TypeScript to be equally covered. Plus I'm after that sweet 100% figure in my coverage report. BUT, from the below report in Travis, we can see that the TypeScript compiler is getting there first and throwing a compilation error.

TSError: ⨯ Unable to compile TypeScript:

dev/test/completeRound.spec.ts:414:28 - error TS2345: Argument of type 'number[]' is not assignable to parameter of type 'number'.

414 expect(() => completeRound([5])).to.throw(TypeError, /a number/);

I don't see a way that I can test the compiled code because that means, well, compiling it, I'm using webpack to compile it which straightaway put it in my ./dist folder, browserifies it and everything else, so I can't see how to import from that, and I don't really want a 2-step process where I compile it then bundle it.

I guess the short version is, I want to test JavaScript error throwing for an error where TypeScript would catch it automatically, but JavaScript wouldn't.

Can anyone help?