diff --git a/ext/io/console/extconf.rb b/ext/io/console/extconf.rb
index 2f363152e3..bbd1235986 100644
--- a/ext/io/console/extconf.rb
+++ b/ext/io/console/extconf.rb
@@ -19,5 +19,8 @@ if ok
   have_func("rb_check_hash_type", "ruby.h")
   have_func("rb_io_get_write_io", "ruby/io.h")
   have_func("rb_cloexec_open", "ruby/io.h")
+  if enable_config("io-console-rb_scan_args-optional-hash", true)
+    $defs << "-DHAVE_RB_SCAN_ARGS_OPTIONAL_HASH=1"
+  end
   create_makefile("io/console")
 end