diff --git a/app/models/homework_common.rb b/app/models/homework_common.rb
index fc2dd3ea4..def3b6b0d 100644
--- a/app/models/homework_common.rb
+++ b/app/models/homework_common.rb
@@ -37,6 +37,7 @@ class HomeworkCommon < ApplicationRecord
 
   validates :name, length: { maximum: 60 }
   validates :description, length: { maximum: 15000 }
+  validates :explanation, length: { maximum: 5000 }
   validates :reference_answer, length: { maximum: 15000 }
 
   # after_update :update_activity