diff --git a/lib/signed-xml.js b/lib/signed-xml.js
index 1b511d3d..b02ff078 100644
--- a/lib/signed-xml.js
+++ b/lib/signed-xml.js
@@ -713,7 +713,7 @@ SignedXml.prototype.computeSignature = function(xml, opts, callback) {
   prefix = opts.prefix;
   attrs = opts.attrs || {};
   location = opts.location || {};
-  existingPrefixes = opts.existingPrefixes || {};
+  var existingPrefixes = opts.existingPrefixes || {};
   // defaults to the root node
   location.reference = location.reference || "/*";
   // defaults to append action
@@ -820,7 +820,7 @@ SignedXml.prototype.computeSignature = function(xml, opts, callback) {
         signatureDoc.insertBefore(self.createSignature(prefix), signedInfoNode.nextSibling)
         self.signatureXml = signatureDoc.toString()
         self.signedXml = doc.toString()
-        callback()
+        callback(null, self)
       }
     })
   }
@@ -1017,4 +1017,4 @@ SignedXml.prototype.getOriginalXmlWithIds = function() {
 
 SignedXml.prototype.getSignedXml = function() {
   return this.signedXml
-}
\ No newline at end of file
+}