diff --git a/src/client.c b/src/client.c
index cb92729..ae2dcef 100644
--- a/src/client.c
+++ b/src/client.c
@@ -1273,7 +1273,7 @@ send_handshake_query(int fd, char *prefix)
 	buf[0] = 0;
 	strncat(buf, prefix, 60); /* 63 - space for 3 CMC bytes */
 	strcat(buf, cmc_dot);
-	strncat(buf, topdomain, sizeof(buf) - strlen(buf));
+	strncat(buf, topdomain, sizeof(buf) - strlen(buf) - 1);
 	send_query(fd, buf);
 }