```typescript
/** Returns the division of `a` by `b`, throwing if either argument is invalid. */
export function safeDivide(a: number, b: number): number {
  /**
   * @throws {Error} If `a` or `b` is not a finite number.
   */
  if (typeof a === 'number' && typeof b === 'number') {
    return a / b;
  } else {
    throw new Error('Both arguments must be numbers.');
  }
}
```