diff --git a/Library/Homebrew/build.rb b/Library/Homebrew/build.rb
index 82e70cf8c1..98924f648c 100755
--- a/Library/Homebrew/build.rb
+++ b/Library/Homebrew/build.rb
@@ -159,7 +159,7 @@ def fixopt f
     f.linked_keg.readlink
   elsif f.prefix.directory?
     f.prefix
-  elsif (kids = f.rack.children).size == 1
+  elsif (kids = f.rack.children).size == 1 and kids.first.directory?
     kids.first
   else
     raise