diff --git a/net/tftp.c b/net/tftp.c
index ea8fea2..84d83ca 100644
--- a/net/tftp.c
+++ b/net/tftp.c
@@ -178,7 +178,7 @@
 		pkt += 5 /*strlen("octet")*/ + 1;
 		strcpy ((char *)pkt, "timeout");
 		pkt += 7 /*strlen("timeout")*/ + 1;
-		sprintf((char *)pkt, "%d", TIMEOUT);
+		sprintf((char *)pkt, "%lu", TIMEOUT);
 #ifdef ET_DEBUG
 		printf("send option \"timeout %s\"\n", (char *)pkt);
 #endif