diff --git a/src/syntax.c b/src/syntax.c
index 4ab87a3a3c28b7b9e901b58a014b5f1d21fcf30d..b17a8d4d0bcd1e9f5aa74efec8ae27d75c44b5fb 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -877,8 +877,8 @@ DEFUN ("modify-syntax-entry", Fmodify_syntax_entry, Smodify_syntax_entry, 2, 3,
   if (*p)
     {
       int len;
-      int character = STRING_CHAR_AND_LENGTH (p, XSTRING (newentry)->size - 1,
-					      len);
+      int character = (STRING_CHAR_AND_LENGTH
+		       (p, STRING_BYTES (XSTRING (newentry)) - 1, len));
       XSETINT (match, character);
       if (XFASTINT (match) == ' ')
 	match = Qnil;