diff --git a/src/bytecode.c b/src/bytecode.c
index 4b909a1b3dc..b0e2ca7a8b2 100644
--- a/src/bytecode.c
+++ b/src/bytecode.c
@@ -306,7 +306,7 @@ If the third argument is incorrect, Emacs may crash.")
   /* Cached address of beginning of string,
      valid if BYTESTR equals STRING_SAVED.  */
   register unsigned char *strbeg;
-  int bytestr_length = XSTRING (bytestr)->size;
+  int bytestr_length = XSTRING (bytestr)->size_byte;
 
   CHECK_STRING (bytestr, 0);
   if (!VECTORP (vector))