diff --git a/ext/hyperll/register_set.c b/ext/hyperll/register_set.c
index 952c8fe..0592941 100644
--- a/ext/hyperll/register_set.c
+++ b/ext/hyperll/register_set.c
@@ -182,6 +182,7 @@ static VALUE rb_register_set_size(VALUE self) {
 
 void Init_hyperll_register_set(void) {
   rb_cRegisterSet = rb_define_class_under(rb_mHyperll, "RegisterSet", rb_cObject);
+  rb_undef_alloc_func(rb_cRegisterSet);
   rb_include_module(rb_cRegisterSet, rb_mEnumerable);
 
   rb_define_singleton_method(rb_cRegisterSet, "new", rb_register_set_new, -1);