In stop-and-wait ARQ why shoukld the receiver always send an acknowledgement message each time it receives a frame with a wrong number sequence?

When the receiver receives a good data packet, it sends an acknowledgement packet to the transmittor with a sexquence number of the NEXT packet it expects. That means, if the Ĩlast received data block has a sequence number 0, then the ACK contains sequence number 1, because the next block the receiver expects, ought to contain seq. number 1.