diff --git a/Library/Homebrew/cmd/postinstall.rb b/Library/Homebrew/cmd/postinstall.rb
index 95bd3f8ef09b62489c584e79bdb3ba05a4006d17..798907339c47848df212adadc4f64f687675e88e 100644
--- a/Library/Homebrew/cmd/postinstall.rb
+++ b/Library/Homebrew/cmd/postinstall.rb
@@ -21,12 +21,10 @@ module Homebrew
       args << "--devel"
     end
 
-    if Sandbox.available? && ARGV.sandbox?
-      Sandbox.print_sandbox_message
-    end
+    Sandbox.print_sandbox_message if Sandbox.formula?(formula)
 
     Utils.safe_fork do
-      if Sandbox.available? && ARGV.sandbox?
+      if Sandbox.formula?(formula)
         sandbox = Sandbox.new
         formula.logs.mkpath
         sandbox.record_log(formula.logs/"sandbox.postinstall.log")