diff --git a/app/models/kuroko2/job_definition.rb b/app/models/kuroko2/job_definition.rb
index 77e93a97..98fb974f 100644
--- a/app/models/kuroko2/job_definition.rb
+++ b/app/models/kuroko2/job_definition.rb
@@ -67,10 +67,12 @@ module PreventMultiStatus
   validate :script_syntax
   validate :validate_number_of_admins
   validates :hipchat_additional_text, length: { maximum: 180 }
-  validates :slack_channel, length: { maximum: 21 }, format: {
-    with: /\A#[^\.\s]+\z/, allow_blank: true,
-    message: ' must start with # and must not include any dots or spaces'
-  }
+  validates :slack_channel,
+    length: { maximum: 22, too_long: ' is too long (maximum is 21 characters without `#` symbol at the head)' },
+    format: {
+      with: /\A#[^\.\s]+\z/, allow_blank: true,
+      message: ' must start with # and must not include any dots or spaces'
+    }
   validates :webhook_url, format: { with: /\A#{URI::regexp(%w(http https))}\z/, allow_blank: true }
 
   def proceed_multi_instance?