Identify the error in this Null Object pattern implementation:

Identify the error in this Null Object pattern implementation:

class NullCache {
  get(key) { return null; }
  set(key, value) { /* do nothing */ }
}
function fetchData(cache, key) {
  const data = cache.get(key);
  if (data) {
    return data;
  } else {
    return 'default';
  }
}
const cache = new NullCache();
console.log(fetchData(cache, 'item'));