diff --git a/lib/overcommit/hook_context/diff.rb b/lib/overcommit/hook_context/diff.rb
index 0c4b97e0..3e9aa568 100644
--- a/lib/overcommit/hook_context/diff.rb
+++ b/lib/overcommit/hook_context/diff.rb
@@ -2,6 +2,8 @@
 
 require 'overcommit/git_repo'
 
+require 'set'
+
 module Overcommit::HookContext
   # Simulates a pre-commit context based on the diff with another git ref.
   #