diff --git a/lib/redmine_custom_workflows/group_patch.rb b/lib/redmine_custom_workflows/group_patch.rb
index bc36610..81087d3 100644
--- a/lib/redmine_custom_workflows/group_patch.rb
+++ b/lib/redmine_custom_workflows/group_patch.rb
@@ -22,7 +22,7 @@ module RedmineCustomWorkflows
                                                lambda { |group, user| Group.users_callback(observable, group, user) }
                                              end
         end
-      end
+      end if CustomWorkflow.table_exists?
     end
 
     module InstanceMethods