Polymorphic Pattern Lab verification question

I worked my way through the lab and got a validation code and passed. However when I compare my answer to the given answer, the 2nd document differs from my answer and I was wondering if I’ve actually done it right? My answer shown below (I won’t post the official answer for the benefit of others)

{
“_id”: “< objectId >”,
“product_id”: “< int >”,
“title”: “< string >”,
“description”: “< string >”,
“authors”: ["< string >"],
“publisher”: “< string >”,
“language”: “< string >”,
“pages”: “< int >”,
“eformats”: ["< string >"],
“format”: “< string >”,
“isbn10”: “< string >”
}