diff --git a/lib/runtime.js b/lib/runtime.js
index 3ab9c68..bdff249 100644
--- a/lib/runtime.js
+++ b/lib/runtime.js
@@ -51,6 +51,6 @@ function angstrom_bigstring_blit_to_bigstring(src, src_off, dst, dst_off, len) {
 //Requires: caml_ba_set_1, string_unsafe_get
 function angstrom_bigstring_blit_from_bytes(src, src_off, dst, dst_off, len) {
   for (var i = 0; i < len; i++) {
-    caml_ba_set_1(dst, dstoff + i, string_unsafe_get(src, src_off + i));
+    caml_ba_set_1(dst, dst_off + i, string_unsafe_get(src, src_off + i));
   }
 }