diff --git a/lib/routes.js b/lib/routes.js
index 79c3fe8..459777a 100644
--- a/lib/routes.js
+++ b/lib/routes.js
@@ -11,7 +11,7 @@ function excludeRoutes(patterns, routes) {
   patterns.forEach(pattern => {
     const minimatch = new Minimatch(pattern)
     minimatch.negate = true
-    routes = routes.filter(({ url }) => minimatch.match(url))
+    routes = (routes || []).filter(({ url }) => minimatch.match(url))
   })
   return routes
 }