diff --git a/lualib-src/sproto/sproto.c b/lualib-src/sproto/sproto.c
index 36b209ceb8..75a4c8a42c 100644
--- a/lualib-src/sproto/sproto.c
+++ b/lualib-src/sproto/sproto.c
@@ -1306,7 +1306,7 @@ sproto_pack(const void * srcv, int srcsz, void * bufferv, int bufsz) {
 			++ff_n;
 			if (ff_n == 256) {
 				if (bufsz >= 0) {
-					write_ff(ff_srcstart, ff_desstart, 256*8);
+					write_ff(ff_srcstart, ff_desstart, srcsz < 256*8 ? srcsz : 256*8);
 				}
 				ff_n = 0;
 			}