diff --git a/lib/builder.js b/lib/builder.js
index c4c39e3..48b4b63 100644
--- a/lib/builder.js
+++ b/lib/builder.js
@@ -59,7 +59,7 @@ function createSitemap(options, routes, base = null, req = null) {
     // Normalize to absolute path
     return {
       ...sitemapOptions,
-      url: join('.', sitemapOptions.url),
+      url: join('.', String(sitemapOptions.url)),
     }
   })
 
diff --git a/lib/cache.js b/lib/cache.js
index a20389e..ef613f9 100644
--- a/lib/cache.js
+++ b/lib/cache.js
@@ -48,7 +48,7 @@ function promisifyRoute(fn) {
   // If routes is a function expecting a callback
   if (fn.length === 1) {
     return new Promise((resolve, reject) => {
-      fn(function(err, routeParams) {
+      fn(function (err, routeParams) {
         if (err) {
           reject(err)
         }
@@ -80,13 +80,16 @@ function joinRoutes(staticRoutes, dynamicRoutes) {
 }
 
 /**
- * Make sure a route is an object with "url" property
+ * Make sure a route is an object with an "url" string property
  *
  * @param   {Object | string} route Route Object or Payload Object or String value
  * @returns {Object} A valid route object
  */
 function ensureIsValidRoute(route) {
-  return typeof route === 'object' ? (route.route ? { url: route.route } : route) : { url: route }
+  route = typeof route === 'object' ? (route.route ? { url: route.route } : route) : { url: route }
+  // force as string
+  route.url = String(route.url)
+  return route
 }
 
 module.exports = { createRoutesCache }