diff --git a/flask/json.py b/flask/json.py
index 875af67e..2437a20c 100644
--- a/flask/json.py
+++ b/flask/json.py
@@ -92,18 +92,22 @@ class JSONDecoder(_json.JSONDecoder):
 
 def _dump_arg_defaults(kwargs):
     """Inject default arguments for dump functions."""
-    kwargs.setdefault('sort_keys', True)
     if current_app:
         kwargs.setdefault('cls', current_app.json_encoder)
         if not current_app.config['JSON_AS_ASCII']:
             kwargs.setdefault('ensure_ascii', False)
         kwargs.setdefault('sort_keys', current_app.config['JSON_SORT_KEYS'])
+    else:
+        kwargs.setdefault('sort_keys', True)
+        kwargs.setdefault('cls', JSONEncoder)
 
 
 def _load_arg_defaults(kwargs):
     """Inject default arguments for load functions."""
     if current_app:
         kwargs.setdefault('cls', current_app.json_decoder)
+    else:
+        kwargs.setdefault('cls', JSONDecoder)
 
 
 def dumps(obj, **kwargs):