diff --git a/Library/Homebrew/extend/ENV/std.rb b/Library/Homebrew/extend/ENV/std.rb
index 0e0ba9846d..41c0861f18 100644
--- a/Library/Homebrew/extend/ENV/std.rb
+++ b/Library/Homebrew/extend/ENV/std.rb
@@ -112,7 +112,7 @@ module Stdenv
   def clang
     super
     replace_in_cflags(/-Xarch_#{Hardware::CPU.arch_32_bit} (-march=\S*)/, '\1')
-    map = Hardware::CPU.optimization_flags
+    map = Hardware::CPU.optimization_flags.dup
     if DevelopmentTools.clang_build_version < 700
       # Clang mistakenly enables AES-NI on plain Nehalem
       map[:nehalem] = "-march=nehalem -Xclang -target-feature -Xclang -aes"