diff --git a/init.rb b/init.rb
index 406f9581..e3e25e8e 100644
--- a/init.rb
+++ b/init.rb
@@ -181,7 +181,7 @@ Redmine::Plugin.register :redmine_dmsf do
         end
         raise 'Not supported image format' unless file.image?
         url = url_for(:controller => :dmsf_files, :action => 'view', :id => file)      
-       if size.include? "%"
+       if size && size.include?('%')
           image_tag(url, :alt => file.title, :width => size, :height => size)
         else
           image_tag(url, :alt => file.title, :size => size)