diff --git a/Library/Homebrew/tap.rb b/Library/Homebrew/tap.rb
index b038d3020d..2d091ba338 100644
--- a/Library/Homebrew/tap.rb
+++ b/Library/Homebrew/tap.rb
@@ -1,3 +1,5 @@
+require "extend/string"
+
 # a {Tap} is used to extend the formulae provided by Homebrew core.
 # Usually, it's synced with a remote git repository. And it's likely
 # a Github repository with the name of `user/homebrew-repo`. In such