A family of Microsoft word processing software products for creating web, email, and print documents.
No, I am not referring to the \d switch. Even if you use that with some meaningful content, but the user deletes that and leaves an empty result, you'll get the errors that are now occurring.
When you use an ASK field, it creates a bookmark - but only if you provide an non-empty response. Your REF fields are generating errors because, when you give no response to the ASK prompt, the bookmark doesn't get created. The solution is to ensure the bookmark exits. You do that by either of the two methods I suggested.