diff --git a/app/models/dmsf_lock.rb b/app/models/dmsf_lock.rb
index a447ef79..2f427311 100644
--- a/app/models/dmsf_lock.rb
+++ b/app/models/dmsf_lock.rb
@@ -1,7 +1,8 @@
 # Redmine plugin for Document Management System "Features"
 #
-# Copyright (C) 2011   Vít Jonáš <vit.jonas@gmail.com>
-# Copyright (C) 2012   Daniel Munn  <dan.munn@munnster.co.uk>
+# Copyright (C) 2011    Vít Jonáš <vit.jonas@gmail.com>
+# Copyright (C) 2012    Daniel Munn  <dan.munn@munnster.co.uk>
+# Copyright (C) 2011-14 Karel Picman <karel.picman@kontorn.com>
 #
 # This program is free software; you can redistribute it and/or
 # modify it under the terms of the GNU General Public License
@@ -17,6 +18,8 @@
 # along with this program; if not, write to the Free Software
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 
+require 'simple_enum'
+
 class DmsfLock < ActiveRecord::Base
   before_create :generate_uuid
   belongs_to :file, :class_name => 'DmsfFile', :foreign_key => 'entity_id'