diff --git a/lib/redmine_slack/issue_patch.rb b/lib/redmine_slack/issue_patch.rb
index 1c9d73b..8f2b4a7 100644
--- a/lib/redmine_slack/issue_patch.rb
+++ b/lib/redmine_slack/issue_patch.rb
@@ -5,7 +5,7 @@ def self.included(base) # :nodoc:
 			base.send(:include, InstanceMethods)
 
 			base.class_eval do
-				unloadable # Send unloadable so it will not be unloaded in development
+				unloadable if respond_to?(:unloadable)  # Send unloadable so it will not be unloaded in development
 				after_create :create_from_issue
 				after_save :save_from_issue
 			end