Unable to lookup or set document body and xattr in single subdoc operation

Unable to get or set full document body and xattr in a single operation, when using gocb.

Attempting to set the path to "" for the document body portion of the SUBDOC_MULTI_LOOKUP or SUBDOC_MULTI_MUTATION operation results in an 'invalid arguments' error, which appears to be gocb's translation of statusCode(0x04).

gocb code to for the attempted upsert:

docFragment, err := bucket.Bucket.MutateIn(k, 0, uint32(exp)).
UpsertEx("sync", xv, gocb.SubdocFlagXattr|gocb.SubdocFlagCreatePath).
 UpsertEx("", v, gocb.SubdocFlagNone).
 Execute()

 Based on my review of the gocb code, it's not complaining about the empty path - the request gets sent to the server successfully.  

I'm getting the same error (invalid arguments) when trying to do a lookup of doc body and XATTR in a single multi_lookup.