Here is the corrected program:
```ts
/** The factorial of n. */
export function factorial(n: number): number {
  if (n < 0 || !Number.isInteger(n)) {
    throw new Error('Argument must be a non-negative integer');
}
  else if (n === 0) {
    return 1;
  } else {
    return n * factorial(n - 1);
  }
}
```