Arduino Ethercard SEND auto ACK

I am using arduino with ethercard and an ENC28j60 chip. In this case arduino works as a TCP server. One of the messages I receive contains the data SETtck: and 4 bytes. In this case I have to send back 4 bytes in the response message:

Part of my code contains the following:

const char Period[4]={XVC_PERIOD&0xFF,
(XVC_PERIOD&0xFF0000) >> 16,
(XVC_PERIOD&0xFF000000)>> 24};

if (strstr((char *)Ethernet::buffer + pos, "settck:") ){
        Serial.println("Recibido comando settck:");
        pos +=7;
        BufferFiller bfill = ether.tcpOffset();

Analyzing with wireshark you can see that 2 packets are sent back, one from an ACK without Data and the other from ACK + Data (RETRANSMISION in wireshark). The valid one would be the second:

How can I prevent the ACK packet from being sent without data.

I also tried the httpServerReply_with_flags and the same thing happens, it always sends an invalid ACK packet.

Thank you.

you could try my EthernetENC library which is a proper arduino networking library for enc28j60