diff --git a/middleman-core/lib/middleman-core/extension.rb b/middleman-core/lib/middleman-core/extension.rb
index b900c430..210ff016 100644
--- a/middleman-core/lib/middleman-core/extension.rb
+++ b/middleman-core/lib/middleman-core/extension.rb
@@ -1,3 +1,4 @@
+require 'forwardable'
 require 'active_support/core_ext/class/attribute'
 require 'middleman-core/configuration'
 require 'middleman-core/contracts'