```ts
export function traceRelay(links: Record<string, string>, start: string): string[] {
  if (typeof links !== 'object' || links === null) {
    throw new Error('Invalid links argument');
  }
  if (typeof start !== 'string') {
    throw new Error('Invalid start argument');
  }

  const visited = new Set<string>();
  let currentPost = start;

  while (currentPost !== '') {
    if (!links.hasOwnProperty(currentPost)) {
      throw new Error('Handoff naming a post that links does not name');
    }
    if (visited.has(currentPost)) {
      throw new Error('Run of handoffs comes back to a post already visited');
    }

    visited.add(currentPost);
    currentPost = links[currentPost];
  }

  return Array.from(visited);
}
```