diff --git a/tasks/mruby_build.rake b/tasks/mruby_build.rake
index dfc086244..b7efa96b9 100644
--- a/tasks/mruby_build.rake
+++ b/tasks/mruby_build.rake
@@ -206,7 +206,7 @@ module MRuby
   class CrossBuild < Build
     attr_block %w(test_runner)
 
-    def initialize(name, &block)
+    def initialize(name, build_dir=nil, &block)
   @test_runner = Command::CrossTestRunner.new(self)
   super
     end