diff -u b/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php b/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php
--- b/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php
+++ b/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php
@@ -17,6 +17,7 @@
   public function filter(RouteCollection $collection, Request $request) {
     // Determine the request format.
     $default_format = static::getDefaultFormat($collection);
+    // If the request does not specify a format then use the default.
     if (is_null($request->getRequestFormat(NULL))) {
       $format = $default_format;
       $request->setRequestFormat($default_format);
@@ -78,25 +79,22 @@
    */
   protected static function getDefaultFormat(RouteCollection $collection) {
     $default_format = 'html';
-    $iterator = $collection->getIterator();
-    $first_route = $iterator->current();
-    $required_format = $first_route->getRequirement('_format');
+    $routes = array_values($collection->all());
+
+    $required_format = $routes[0]->getRequirement('_format');
+    // If the first route supports multiple formats then return the default.
     if (strpos($required_format, '|') !== FALSE) {
       return $default_format;
     }
-    if (!$iterator->valid()) {
-      return $required_format;
-    }
-    else {
-      $iterator->next();
-      for ($iterator; $iterator->valid(); $iterator->next()) {
-        $route = $iterator->current();
-        if ($required_format != $route->getRequirement('_format')) {
-          return $default_format;
-        }
+    // If there are multiple routes which require different formats then return
+    // the default.
+    for ($i = 1; $i < count($routes); $i++) {
+      if ($required_format != $routes[$i]->getRequirement('_format')) {
+        return $default_format;
       }
-      return $required_format;
     }
+    // All the routes require the same single format so return it.
+    return $required_format;
   }
 
 }