diff --git a/lib/security.js b/lib/security.js
index 8d34a78..e011946 100644
--- a/lib/security.js
+++ b/lib/security.js
@@ -14,7 +14,7 @@ class PDFSecurity {
       if (!info.hasOwnProperty(key)) {
         continue;
       }
-      infoStr += `${key}: ${info[key]}\n`;
+      infoStr += `${key}: ${info[key].valueOf()}\n`;
     }
 
     return wordArrayToBuffer(CryptoJS.MD5(infoStr));