SubDoc operation

I am using this code to fetch subDocumnent from a specific document

DocumentFragment result= bucket.async().lookupIn(docId).get(subDocId).execute() . toBlocking().singleOrDefault(null);

I am not sure why but

result.rawContent(subDocId)

returns null, whereas

result.content(subDocId)

returns the proper value.

Can anyone point to me what might be causing this issue?