diff --git a/README.md b/README.md
index 649cf5b8..bad57d54 100644
--- a/README.md
+++ b/README.md
@@ -117,7 +117,14 @@ type Profile = {
   * `decryptionPvk`: optional private key that will be used to attempt to decrypt any encrypted assertions that are received
   * `signatureAlgorithm`: optionally set the signature algorithm for signing requests, valid values are 'sha1' (default), 'sha256', or 'sha512'
   * `digestAlgorithm`: optionally set the digest algorithm used to provide a digest for the signed data object, valid values are 'sha1' (default), 'sha256', or 'sha512'
-  * `xmlSignatureTransforms`: optionally set an array of signature transforms to be used in HTTP-POST signatures. By default this is `[ 'http://www.w3.org/2000/09/xmldsig#enveloped-signature', 'http://www.w3.org/2001/10/xml-exc-c14n#' ]`
+  * `xmlSignatureTransforms`: optionally set an array of signature transforms to be used in HTTP-POST signatures. By 
+  default this is `[ 'http://www.w3.org/2000/09/xmldsig#enveloped-signature', 'http://www.w3.org/2001/10/xml-exc-c14n#' ]`
+  * `attrValueMapper`: optionally set a function that maps attributes values into plain strings. By default this is:
+    ```js
+    attrValueMapper = function (value) {
+      return typeof value === 'string' ? value : value._;
+    };
+     ```
  * **Additional SAML behaviors**
   * `additionalParams`: dictionary of additional query params to add to all requests; if an object with this key is passed to `authenticate`, the dictionary of additional query params will be appended to those present on the returned URL, overriding any specified by initialization options' additional parameters (`additionalParams`, `additionalAuthorizeParams`, and `additionalLogoutParams`)
   * `additionalAuthorizeParams`: dictionary of additional query params to add to 'authorize' requests
diff --git a/lib/passport-saml/saml.js b/lib/passport-saml/saml.js
index 519d22ae..7c7c30a2 100644
--- a/lib/passport-saml/saml.js
+++ b/lib/passport-saml/saml.js
@@ -80,6 +80,12 @@ SAML.prototype.initialize = function (options) {
     options.signatureAlgorithm = 'sha1';
   }
 
+  if(!options.attrValueMapper) {
+    options.attrValueMapper = function (value) {
+      return typeof value === 'string' ? value : value._;
+    };
+  }
+
   /**
    * List of possible values:
    * - exact : Assertion context must exactly match a context in the list
@@ -1008,9 +1014,7 @@ SAML.prototype.processValidlySignedAssertion = function(xml, samlResponseXml, in
                                                   .map(attr => attr.Attribute)
                                );
 
-      var attrValueMapper = function(value) {
-        return typeof value === 'string' ? value : value._;
-      };
+      var attrValueMapper = this.options.attrValueMapper;
 
       if (attributes) {
         attributes.forEach(attribute => {