diff --git a/zephyr/lib/actions.py b/zephyr/lib/actions.py
index b1aa7abef5..715543b8f1 100644
--- a/zephyr/lib/actions.py
+++ b/zephyr/lib/actions.py
@@ -965,6 +965,8 @@ def do_update_message(user_profile, message_id, subject, content):
                 first_rendered_content = old_edit_history_event['prev_rendered_content']
 
     if content is not None:
+        if len(content) > MAX_MESSAGE_LENGTH:
+            raise JsonableError("Message too long")
         rendered_content = bugdown.convert(content, message.sender.realm.domain)
         if rendered_content is None:
             raise JsonableError("We were unable to render your updated message")
@@ -985,6 +987,8 @@ def do_update_message(user_profile, message_id, subject, content):
         event["rendered_content"] = rendered_content
 
     if subject is not None:
+        if len(subject) > MAX_SUBJECT_LENGTH:
+            raise JsonableError("Subject too long")
         event["orig_subject"] = message.subject
         message.subject = subject
         event["subject"] = subject