diff --git a/src/tftp.c b/src/tftp.c
index dc4aa85..5e4a32a 100644
--- a/src/tftp.c
+++ b/src/tftp.c
@@ -346,14 +346,15 @@ void tftp_request(struct listener *listen, time_t now)
 	    {
 	      if ((opt = next(&p, end)) && !option_bool(OPT_TFTP_NOBLOCK))
 		{
+		  /* 32 bytes for IP, UDP and TFTP headers, 52 bytes for IPv6 */
+		  int overhead = (listen->family == AF_INET) ? 32 : 52;
 		  transfer->blocksize = atoi(opt);
 		  if (transfer->blocksize < 1)
 		    transfer->blocksize = 1;
 		  if (transfer->blocksize > (unsigned)daemon->packet_buff_sz - 4)
 		    transfer->blocksize = (unsigned)daemon->packet_buff_sz - 4;
-		  /* 32 bytes for IP, UDP and TFTP headers */
-		  if (mtu != 0 && transfer->blocksize > (unsigned)mtu - 32)
-		    transfer->blocksize = (unsigned)mtu - 32;
+		  if (mtu != 0 && transfer->blocksize > (unsigned)mtu - overhead)
+		    transfer->blocksize = (unsigned)mtu - overhead;
 		  transfer->opt_blocksize = 1;
 		  transfer->block = 0;
 		}