Doe it literally check for the specific solution?

Writing

    to = append(to, ctrl)

which is functionally equivalent and, in my personal opinion, with clearer intent (ctrl = 0 at that point in the code), returns "incorrect".

In fact, it seems that any placeholder value should work - as it is always overwritten by the final value of ctrl for a given set of bytes at the end; however, the checker rejects this.