1 files changed, 13 insertions, 1 deletions
diff --git a/src/class.c b/src/class.c
index b81859ab8..7097a593a 100644
--- a/src/class.c
+++ b/src/class.c
@@ -1783,9 +1783,21 @@ mod_define_method(mrb_state *mrb, mrb_value self)
   struct RClass *c = mrb_class_ptr(self);
   struct RProc *p;
   mrb_sym mid;
+  mrb_value proc = mrb_undef_value();
   mrb_value blk;
 
-  mrb_get_args(mrb, "n&", &mid, &blk);
+  mrb_get_args(mrb, "n|o&", &mid, &proc, &blk);
+  switch (mrb_type(proc)) {
+    case MRB_TT_PROC:
+      blk = proc;
+      break;
+    case MRB_TT_UNDEF:
+      /* ignored */
+      break;
+    default:
+      mrb_raisef(mrb, E_TYPE_ERROR, "wrong argument type %S (expected Proc)", mrb_obj_value(mrb_obj_class(mrb, proc)));
+      break;
+  }
   if (mrb_nil_p(blk)) {
     mrb_raise(mrb, E_ARGUMENT_ERROR, "no block given");
   }