- // ISO 14443 APDU frame: PCB [CID] [NAD] APDU CRC PCB=0x02
- real_cmd[0] = 0x02; // bnr,nad,cid,chn=0; i-block(0x00)
- // put block number into the PCB
- real_cmd[0] |= iso14_pcb_blocknum;
- memcpy(real_cmd + 1, cmd, cmd_len);
+ if (cmd_len) {
+ // ISO 14443 APDU frame: PCB [CID] [NAD] APDU CRC PCB=0x02
+ real_cmd[0] = 0x02; // bnr,nad,cid,chn=0; i-block(0x00)
+ // put block number into the PCB
+ real_cmd[0] |= iso14_pcb_blocknum;
+ memcpy(real_cmd + 1, cmd, cmd_len);
+ } else {
+ // R-block. ACK
+ real_cmd[0] = 0xA2; // r-block + ACK
+ real_cmd[0] |= iso14_pcb_blocknum;
+ }