diff --git a/Library/Homebrew/superenv.rb b/Library/Homebrew/superenv.rb
index c8c21e1720..58a801ca53 100644
--- a/Library/Homebrew/superenv.rb
+++ b/Library/Homebrew/superenv.rb
@@ -17,6 +17,30 @@ module Superenv
   def self.extended(base)
     base.keg_only_deps = []
     base.deps = []
+
+    # Many formula assume that CFLAGS etc. will not be nil. This should be
+    # a safe hack to prevent that exception cropping up. Main consequence of
+    # this is that self['CFLAGS'] is never nil even when it is which can break
+    # if checks, but we don't do such a check in our code. Redefinition must be
+    # done on the singleton class, because in MRI all ENV methods are defined
+    # on its singleton class, precluding the use of extend.
+    class << base
+      def [] key
+        if has_key? key
+          fetch(key)
+        elsif %w{CPPFLAGS CFLAGS LDFLAGS}.include? key
+          class << (a = "")
+            attr_accessor :key
+            def + value
+              ENV[key] = value
+            end
+            alias_method '<<', '+'
+          end
+          a.key = key
+          a
+        end
+      end
+    end
   end
 
   def self.bin
@@ -279,26 +303,6 @@ module Superenv
     self['MAKEFLAGS'] =~ /-\w*j(\d)+/
     [$1.to_i, 1].max
   end
-
-  # Many formula assume that CFLAGS etc. will not be nil.
-  # This should be a safe hack to prevent that exception cropping up.
-  # Main consqeuence of this is that self['CFLAGS'] is never nil even when it
-  # is which can break if checks, but we don't do such a check in our code.
-  def [] key
-    if has_key? key
-      fetch(key)
-    elsif %w{CPPFLAGS CFLAGS LDFLAGS}.include? key
-      class << (a = "")
-        attr_accessor :key
-        def + value
-          ENV[key] = value
-        end
-        alias_method '<<', '+'
-      end
-      a.key = key
-      a
-    end
-  end
 end