diff --git a/source/Ox/js/Format.js b/source/Ox/js/Format.js
index b88a0b53..d151dc1a 100644
--- a/source/Ox/js/Format.js
+++ b/source/Ox/js/Format.js
@@ -653,13 +653,13 @@ Ox.formatISBN = function(isbn, length) {
     var ret = '';
     function getCheckDigit(isbn) {
         var mod = isbn.length == 10 ? 11 : 10
-        return (mod - Ox.sum(
+        return (Ox.mod(mod - Ox.sum(
             isbn.slice(0, -1).split('').map(function(digit, index) {
                 return isbn.length == 10
                     ? parseInt(digit) * (10 - index)
                     : parseInt(digit) * (index % 2 == 0 ? 1 : 3); 
             })
-        ) % mod + '').replace('10', 'X');
+        ), mod) + '').replace('10', 'X');
     }
     isbn = isbn.toUpperCase().replace(/[^\dX]/g, '');
     if (isbn.length == 10) {