diff --git a/lib/redmine/helpers/diff.rb b/lib/redmine/helpers/diff.rb
index 537bfa05d..d580f689c 100644
--- a/lib/redmine/helpers/diff.rb
+++ b/lib/redmine/helpers/diff.rb
@@ -15,6 +15,8 @@
 # along with this program; if not, write to the Free Software
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 
+require 'diff'
+
 module Redmine
   module Helpers
     class Diff