diff --git a/rest_framework/routers.py b/rest_framework/routers.py
index 37c79d23b..f28432d04 100644
--- a/rest_framework/routers.py
+++ b/rest_framework/routers.py
@@ -284,10 +284,10 @@ class DefaultRouter(SimpleRouter):
         class APIRoot(views.APIView):
             _ignore_model_permissions = True
 
-            def get(self, request, format=None, *args, **kwargs):
+            def get(self, request, *args, **kwargs):
                 ret = {}
                 for key, url_name in api_root_dict.items():
-                    ret[key] = reverse(url_name, request=request, format=format)
+                    ret[key] = reverse(url_name, request=request, format=kwargs.get('format'))
                 return Response(ret)
 
         return APIRoot.as_view()