Hiki Error

cannot load such file -- docdiff/difference (LoadError)
/usr/local/lib/ruby/site_ruby/2.6/rubygems/core_ext/kernel_require.rb:54:in `require'
/usr/local/lib/ruby/site_ruby/2.6/rubygems/core_ext/kernel_require.rb:54:in `require'
/home/richmam/public_html/hiki/hiki/util.rb:9:in `<top (required)>'
/usr/local/lib/ruby/site_ruby/2.6/rubygems/core_ext/kernel_require.rb:54:in `require'
/usr/local/lib/ruby/site_ruby/2.6/rubygems/core_ext/kernel_require.rb:54:in `require'
/home/richmam/public_html/hiki/hiki/config.rb:18:in `<top (required)>'
/usr/local/lib/ruby/site_ruby/2.6/rubygems/core_ext/kernel_require.rb:54:in `require'
/usr/local/lib/ruby/site_ruby/2.6/rubygems/core_ext/kernel_require.rb:54:in `require'
hiki.cgi:23:in `<main>'