diff --git a/spec/default.mspec b/spec/default.mspec
index bd2d736586..f9392e6da2 100644
--- a/spec/default.mspec
+++ b/spec/default.mspec
@@ -1,6 +1,7 @@
 # -*- ruby -*-
 load "./rbconfig.rb"
 load File.dirname(__FILE__) + '/rubyspec/default.mspec'
+OBJDIR = File.expand_path("spec/rubyspec/optional/capi/ext")
 class MSpecScript
   builddir = Dir.pwd
   srcdir = ENV['SRCDIR']