diff --git a/Library/Homebrew/download_strategy.rb b/Library/Homebrew/download_strategy.rb
index 9a74ff59da..e60b879b69 100644
--- a/Library/Homebrew/download_strategy.rb
+++ b/Library/Homebrew/download_strategy.rb
@@ -190,7 +190,7 @@ class GitDownloadStrategy <AbstractDownloadStrategy
         end
       end
       # http://stackoverflow.com/questions/160608/how-to-do-a-git-export-like-svn-export
-      safe_system 'git', 'checkout-index', '-af', "--prefix=#{dst}/"
+      safe_system 'git', 'checkout-index', '-a', '-f', "--prefix=#{dst}/"
     end
   end
 end