diff --git a/src/ChainRouter.php b/src/ChainRouter.php
index 313baa36..5715ab07 100644
--- a/src/ChainRouter.php
+++ b/src/ChainRouter.php
@@ -98,7 +98,7 @@ public function add($router, $priority = 0)
      */
     public function all()
     {
-        if (0 === count($this->sortedRouters)) {
+        if (null === $this->sortedRouters || 0 === count($this->sortedRouters)) {
             $this->sortedRouters = $this->sortRouters();
 
             // setContext() is done here instead of in add() to avoid fatal errors when clearing and warming up caches