diff --git a/lib/rb/Rakefile b/lib/rb/Rakefile
index 5e5e5ac..b509281 100644
--- a/lib/rb/Rakefile
+++ b/lib/rb/Rakefile
@@ -54,7 +54,7 @@
 
   task :'flat_spec' do
     dir = File.dirname(__FILE__) + '/spec'
-    mkdir_p("#{dir}/gen-rb/flat")
+    FileUtils.mkdir_p("#{dir}/gen-rb/flat")
     sh THRIFT, '--gen', 'rb', '--recurse', '-out', "#{dir}/gen-rb/flat", "#{dir}/ThriftNamespacedSpec.thrift"
   end