diff --git a/lib/attribute.js b/lib/attribute.js
index 3803ff6..0d1668d 100644
--- a/lib/attribute.js
+++ b/lib/attribute.js
@@ -185,7 +185,9 @@ Attribute.prototype.toString = function () {
 function formatGuid(format, data) {
   for (var i = 0; i < data.length; i++) {
     var re = new RegExp('\\{' + i + '\\}', 'g');
-    format = format.replace(re, data[i].toString(16));
+    // Leading 0 is needed if value of data[i] is less than 16 (of 10 as hex). 
+    var dataStr = data[i].toString(16);
+    format = format.replace(re, data[i] >= 16 ? dataStr : '0' + dataStr);
   }
   return format;
-}
\ No newline at end of file
+}