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

  const posts = Object.keys(links);
  if (!posts.includes(start)) {
    throw new Error('Invalid start post');
  }

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

  while (currentPost !== '') {
    if (visited.has(currentPost)) {
      throw new Error('Watch comes back to a post already visited');
    }

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

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