diff --git a/armsrc/mifarecmd.c b/armsrc/mifarecmd.c
index decaf39fb..bfa6bb1cd 100644
--- a/armsrc/mifarecmd.c
+++ b/armsrc/mifarecmd.c
@@ -370,7 +370,7 @@ void MifareUReadCard(uint8_t arg0, uint16_t arg1, uint8_t arg2, uint8_t *datain)
 
     countblocks *= 4;
 
-    reply_mix(CMD_ACK, 1, countblocks, BigBuf_max_traceLen(), 0, 0);
+    reply_mix(CMD_ACK, 1, countblocks, dataout - BigBuf_get_addr(), 0, 0);
     FpgaWriteConfWord(FPGA_MAJOR_MODE_OFF);
     LEDsoff();
     BigBuf_free();