diff --git a/django/contrib/messages/storage/cookie.py b/django/contrib/messages/storage/cookie.py
index 619c69249e..258d491bdb 100644
--- a/django/contrib/messages/storage/cookie.py
+++ b/django/contrib/messages/storage/cookie.py
@@ -33,6 +33,9 @@ class MessageDecoder(json.JSONDecoder):
     def process_messages(self, obj):
         if isinstance(obj, list) and obj:
             if obj[0] == MessageEncoder.message_key:
+                if len(obj) == 3:
+                    # Compatibility with previously-encoded messages
+                    return Message(*obj[1:])
                 if obj[1]:
                     obj[3] = mark_safe(obj[3])
                 return Message(*obj[2:])