diff --git a/lib/utils.js b/lib/utils.js
index 4e5cc4c..9121b6f 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -43,9 +43,9 @@ module.exports = {
       getBuiltinNumFmtId = function (nf) {
         var builtin_nfs;
         builtin_nfs = {
-          General: 0,
+          'General': 0,
           '': 0,
-          0: 1,
+          '0': 1,
           '0.00': 2,
           '#,##0': 3,
           '#,##0.00': 4,
@@ -74,7 +74,7 @@ module.exports = {
           'mmss.0': 47,
           '##0.0E+0': 48,
           '@': 49,
-          t0: 59,
+          't0': 59,
           't0.00': 60,
           't#,##0': 61,
           't#,##0.00': 62,