diff --git a/lib/ymodem.c b/lib/ymodem.c
index d44a80c..13c8e96 100644
--- a/lib/ymodem.c
+++ b/lib/ymodem.c
@@ -248,6 +248,7 @@ retry:
     rx->errors = 0;
     rx->seq    = 1;
 
+    recv_bytes = 0;
     /* start to recv file contents */
     fp = &rx->data[PACKET_HEADER];
     for (;;) {
@@ -311,7 +312,12 @@ retry:
             }
 
             if (rx->seq != rx->data[PACKET_SEQNO_INDEX]) {
-                put_byte(uart, NAK);
+                if (rx->data[PACKET_SEQNO_INDEX] == 0 && recv_bytes == 0) { /* YMODEM-1K only ? */
+                    put_byte(uart, ACK);
+                    put_byte(uart, WANTCRC);
+                } else {
+                    put_byte(uart, NAK);
+                }
 #ifdef YMODEM_DEBUG
                 printf("data phase: seq mismatch %d, expected %d\n", rx->data[PACKET_SEQNO_INDEX], rx->seq);
 #endif
diff --git a/lib/ymodem.h b/lib/ymodem.h
index fc14516..d5faec5 100644
--- a/lib/ymodem.h
+++ b/lib/ymodem.h
@@ -26,6 +26,7 @@ extern "C" {
 #define NAK                     (0x15)  /* negative acknowledge */
 #define CA                      (0x18)  /* two of these in succession aborts transfer */
 #define WANTCRC                 (0x43)  /* 'C' == 0x43, request 16-bit CRC */
+#define WANTG                   (0x47)  /* 'G' == 0x47 */
 
 #define ABORT1                  (0x41)  /* 'A' == 0x41, abort by user */
 #define ABORT2                  (0x61)  /* 'a' == 0x61, abort by user */