diff --git a/Library/Homebrew/cmd/postinstall.rb b/Library/Homebrew/cmd/postinstall.rb
index 71b71b3f5a9463d94b88171d29429260664fdf4b..e0bd8e31b1bd3e12381ec0c1361de5b9b8f190f6 100644
--- a/Library/Homebrew/cmd/postinstall.rb
+++ b/Library/Homebrew/cmd/postinstall.rb
@@ -36,7 +36,7 @@ module Homebrew
         sandbox.allow_write_cellar(formula)
         sandbox.allow_write_xcode
         sandbox.allow_write_path HOMEBREW_PREFIX
-        sandbox.deny_write_homebrew_library
+        sandbox.deny_write_homebrew_repository
         sandbox.exec(*args)
       else
         exec(*args)
diff --git a/Library/Homebrew/sandbox.rb b/Library/Homebrew/sandbox.rb
index a7fc99d4a223e7c9ce650eb04ac5c980bb963cef..4d0709cb4b286eea93df218243f4eb032c6bf3bc 100644
--- a/Library/Homebrew/sandbox.rb
+++ b/Library/Homebrew/sandbox.rb
@@ -84,10 +84,14 @@ class Sandbox
     allow_write_path formula.logs
   end
 
-  def deny_write_homebrew_library
-    deny_write_path HOMEBREW_LIBRARY
-    deny_write_path HOMEBREW_REPOSITORY/".git"
+  def deny_write_homebrew_repository
     deny_write HOMEBREW_BREW_FILE
+    if HOMEBREW_PREFIX.to_s != HOMEBREW_REPOSITORY.to_s
+      deny_write_path HOMEBREW_REPOSITORY
+    else
+      deny_write_path HOMEBREW_LIBRARY
+      deny_write_path HOMEBREW_REPOSITORY/".git"
+    end
   end
 
   def exec(*args)