diff --git a/sbotc.c b/sbotc.c
index f0630b4..73ae928 100644
--- a/sbotc.c
+++ b/sbotc.c
@@ -34,7 +34,7 @@
 #define BOXS_MAXLEN 4096
 
 #define write_buf(fd, buf) \
-	write_all(fd, buf, sizeof(buf))
+	write_all(fd, buf, sizeof(buf)-1)
 
 struct boxs_header {
 	uint16_t len;