diff --git a/mrbgems/mruby-pack/src/pack.c b/mrbgems/mruby-pack/src/pack.c
index 26c2890b0..08db9091b 100644
--- a/mrbgems/mruby-pack/src/pack.c
+++ b/mrbgems/mruby-pack/src/pack.c
@@ -921,7 +921,7 @@ unpack_bstr(mrb_state *mrb, const void *src, int slen, mrb_value ary, int count,
 
   const char *sptr0 = (const char*)src;
   const char *sptr = sptr0;
-  if (count == -1)
+  if (count == -1 || count > slen * 8)
     count = slen * 8;
 
   mrb_value dst = mrb_str_new(mrb, NULL, count);