diff --git a/Library/Homebrew/os/mac.rb b/Library/Homebrew/os/mac.rb
index eb9d011d27..8aae29c9d3 100644
--- a/Library/Homebrew/os/mac.rb
+++ b/Library/Homebrew/os/mac.rb
@@ -210,6 +210,7 @@ module OS
       "6.3"   => { :clang => "6.1", :clang_build => 602 },
       "6.3.1" => { :clang => "6.1", :clang_build => 602 },
       "6.3.2" => { :clang => "6.1", :clang_build => 602 },
+      "6.4"   => { :clang => "6.1", :clang_build => 602 },
       "7.0"   => { :clang => "7.0", :clang_build => 700 },
     }