diff --git a/src/client-oauth2.js b/src/client-oauth2.js
index 995c6af..444bc34 100644
--- a/src/client-oauth2.js
+++ b/src/client-oauth2.js
@@ -308,7 +308,7 @@ function ClientOAuth2Token (client, data) {
 ClientOAuth2Token.prototype.expiresIn = function (duration) {
   if (!isNaN(duration)) {
     this.expires = new Date()
-    this.expires.setSeconds(this.expires.getSeconds() + duration)
+    this.expires.setSeconds(this.expires.getSeconds() + Number(duration))
   } else {
     this.expires = undefined
   }