diff --git a/Library/Homebrew/requirements.rb b/Library/Homebrew/requirements.rb
index 881ac4ad18..7b70508f60 100644
--- a/Library/Homebrew/requirements.rb
+++ b/Library/Homebrew/requirements.rb
@@ -59,7 +59,7 @@ class X11Dependency < Requirement
 
   fatal true
 
-  env { x11 }
+  env { ENV.x11 }
 
   def initialize(name="x11", *tags)
     tags.flatten!