ExchangeParallel return zero resp

when i use ExchangeParallel to query a domain like the following code:
dnsReply, dnsResolved, err := upstream.ExchangeParallel(Ups, &query01)

but some situation bad happens:

the err is nil, the dnsReply and dnsResolved is not nil, but the anser in dnsReplay is empty like the picture:

so i think this is a bug, and i advise to fix it