mrb_init_class(mrb_state *mrb)
{
  struct RClass *bob;           /* BasicObject */
  struct RClass *obj;           /* Object */
  struct RClass *mod;           /* Module */
  struct RClass *cls;           /* Class */

  /* boot class hierarchy */
  bob = boot_defclass(mrb, 0);
  obj = boot_defclass(mrb, bob); mrb->object_class = obj;
  mod = boot_defclass(mrb, obj); mrb->module_class = mod;/* obj -> mod */
  cls = boot_defclass(mrb, mod); mrb->class_class = cls; /* obj -> cls */
  /* fix-up loose ends */
  bob->c = obj->c = mod->c = cls->c = cls;
  make_metaclass(mrb, bob);
  make_metaclass(mrb, obj);
  make_metaclass(mrb, mod);
  make_metaclass(mrb, cls);

  /* name basic classes */
  mrb_define_const(mrb, bob, "BasicObject", mrb_obj_value(bob));
  mrb_define_const(mrb, obj, "BasicObject", mrb_obj_value(bob));
  mrb_define_const(mrb, obj, "Object",      mrb_obj_value(obj));
  mrb_define_const(mrb, obj, "Module",      mrb_obj_value(mod));
  mrb_define_const(mrb, obj, "Class",       mrb_obj_value(cls));

  /* name each classes */
  mrb_class_name_class(mrb, NULL, bob, mrb_intern_lit(mrb, "BasicObject"));
  mrb_class_name_class(mrb, NULL, obj, mrb_intern_lit(mrb, "Object")); /* 15.2.1 */
  mrb_class_name_class(mrb, NULL, mod, mrb_intern_lit(mrb, "Module")); /* 15.2.2 */
  mrb_class_name_class(mrb, NULL, cls, mrb_intern_lit(mrb, "Class"));  /* 15.2.3 */

  mrb->proc_class = mrb_define_class(mrb, "Proc", mrb->object_class);  /* 15.2.17 */
  MRB_SET_INSTANCE_TT(mrb->proc_class, MRB_TT_PROC);

  MRB_SET_INSTANCE_TT(cls, MRB_TT_CLASS);
  mrb_define_method(mrb, bob, "initialize",              mrb_bob_init,             MRB_ARGS_NONE());
  mrb_define_method(mrb, bob, "!",                       mrb_bob_not,              MRB_ARGS_NONE());
  mrb_define_method(mrb, bob, "==",                      mrb_obj_equal_m,          MRB_ARGS_REQ(1)); /* 15.3.1.3.1  */
  mrb_define_method(mrb, bob, "!=",                      mrb_obj_not_equal_m,      MRB_ARGS_REQ(1));
  mrb_define_method(mrb, bob, "__id__",                  mrb_obj_id_m,             MRB_ARGS_NONE()); /* 15.3.1.3.3  */
  mrb_define_method(mrb, bob, "__send__",                mrb_f_send,               MRB_ARGS_ANY());  /* 15.3.1.3.4  */
  mrb_define_method(mrb, bob, "instance_eval",           mrb_obj_instance_eval,    MRB_ARGS_ANY());  /* 15.3.1.3.18 */

  mrb_define_class_method(mrb, cls, "new",               mrb_class_new_class,      MRB_ARGS_OPT(1));
  mrb_define_method(mrb, cls, "superclass",              mrb_class_superclass,     MRB_ARGS_NONE()); /* 15.2.3.3.4 */
  mrb_define_method(mrb, cls, "new",                     mrb_instance_new,         MRB_ARGS_ANY());  /* 15.2.3.3.3 */
  mrb_define_method(mrb, cls, "initialize",              mrb_class_initialize,     MRB_ARGS_OPT(1)); /* 15.2.3.3.1 */
  mrb_define_method(mrb, cls, "inherited",               mrb_bob_init,             MRB_ARGS_REQ(1));

  MRB_SET_INSTANCE_TT(mod, MRB_TT_MODULE);
  mrb_define_method(mrb, mod, "class_variable_defined?", mrb_mod_cvar_defined,     MRB_ARGS_REQ(1)); /* 15.2.2.4.16 */
  mrb_define_method(mrb, mod, "class_variable_get",      mrb_mod_cvar_get,         MRB_ARGS_REQ(1)); /* 15.2.2.4.17 */
  mrb_define_method(mrb, mod, "class_variable_set",      mrb_mod_cvar_set,         MRB_ARGS_REQ(2)); /* 15.2.2.4.18 */
  mrb_define_method(mrb, mod, "extend_object",           mrb_mod_extend_object,    MRB_ARGS_REQ(1)); /* 15.2.2.4.25 */
  mrb_define_method(mrb, mod, "extended",                mrb_bob_init,             MRB_ARGS_REQ(1)); /* 15.2.2.4.26 */
  mrb_define_method(mrb, mod, "prepended",               mrb_bob_init,             MRB_ARGS_REQ(1));
  mrb_define_method(mrb, mod, "prepend_features",        mrb_mod_prepend_features, MRB_ARGS_REQ(1));
  mrb_define_method(mrb, mod, "include?",                mrb_mod_include_p,        MRB_ARGS_REQ(1)); /* 15.2.2.4.28 */
  mrb_define_method(mrb, mod, "append_features",         mrb_mod_append_features,  MRB_ARGS_REQ(1)); /* 15.2.2.4.10 */
  mrb_define_method(mrb, mod, "class_eval",              mrb_mod_module_eval,      MRB_ARGS_ANY());  /* 15.2.2.4.15 */
  mrb_define_method(mrb, mod, "included",                mrb_bob_init,             MRB_ARGS_REQ(1)); /* 15.2.2.4.29 */
  mrb_define_method(mrb, mod, "included_modules",        mrb_mod_included_modules, MRB_ARGS_NONE()); /* 15.2.2.4.30 */
  mrb_define_method(mrb, mod, "initialize",              mrb_mod_initialize,       MRB_ARGS_NONE()); /* 15.2.2.4.31 */
  mrb_define_method(mrb, mod, "instance_methods",        mrb_mod_instance_methods, MRB_ARGS_ANY());  /* 15.2.2.4.33 */
  mrb_define_method(mrb, mod, "method_defined?",         mrb_mod_method_defined,   MRB_ARGS_REQ(1)); /* 15.2.2.4.34 */
  mrb_define_method(mrb, mod, "module_eval",             mrb_mod_module_eval,      MRB_ARGS_ANY());  /* 15.2.2.4.35 */
  mrb_define_method(mrb, mod, "module_function",         mrb_mod_module_function,  MRB_ARGS_ANY());
  mrb_define_method(mrb, mod, "private",                 mrb_mod_dummy_visibility, MRB_ARGS_ANY());  /* 15.2.2.4.36 */
  mrb_define_method(mrb, mod, "protected",               mrb_mod_dummy_visibility, MRB_ARGS_ANY());  /* 15.2.2.4.37 */
  mrb_define_method(mrb, mod, "public",                  mrb_mod_dummy_visibility, MRB_ARGS_ANY());  /* 15.2.2.4.38 */
  mrb_define_method(mrb, mod, "remove_class_variable",   mrb_mod_remove_cvar,      MRB_ARGS_REQ(1)); /* 15.2.2.4.39 */
  mrb_define_method(mrb, mod, "remove_method",           mrb_mod_remove_method,    MRB_ARGS_ANY());  /* 15.2.2.4.41 */
  mrb_define_method(mrb, mod, "method_removed",          mrb_bob_init,             MRB_ARGS_REQ(1));
  mrb_define_method(mrb, mod, "attr_reader",             mrb_mod_attr_reader,      MRB_ARGS_ANY());  /* 15.2.2.4.13 */
  mrb_define_method(mrb, mod, "attr_writer",             mrb_mod_attr_writer,      MRB_ARGS_ANY());  /* 15.2.2.4.14 */
  mrb_define_method(mrb, mod, "to_s",                    mrb_mod_to_s,             MRB_ARGS_NONE());
  mrb_define_method(mrb, mod, "inspect",                 mrb_mod_to_s,             MRB_ARGS_NONE());
  mrb_define_method(mrb, mod, "alias_method",            mrb_mod_alias,            MRB_ARGS_ANY());  /* 15.2.2.4.8 */
  mrb_define_method(mrb, mod, "ancestors",               mrb_mod_ancestors,        MRB_ARGS_NONE()); /* 15.2.2.4.9 */
  mrb_define_method(mrb, mod, "undef_method",            mrb_mod_undef,            MRB_ARGS_ANY());  /* 15.2.2.4.41 */
  mrb_define_method(mrb, mod, "const_defined?",          mrb_mod_const_defined,    MRB_ARGS_ARG(1,1)); /* 15.2.2.4.20 */
  mrb_define_method(mrb, mod, "const_get",               mrb_mod_const_get,        MRB_ARGS_REQ(1)); /* 15.2.2.4.21 */
  mrb_define_method(mrb, mod, "const_set",               mrb_mod_const_set,        MRB_ARGS_REQ(2)); /* 15.2.2.4.23 */
  mrb_define_method(mrb, mod, "constants",               mrb_mod_constants,        MRB_ARGS_OPT(1)); /* 15.2.2.4.24 */
  mrb_define_method(mrb, mod, "remove_const",            mrb_mod_remove_const,     MRB_ARGS_REQ(1)); /* 15.2.2.4.40 */
  mrb_define_method(mrb, mod, "const_missing",           mrb_mod_const_missing,    MRB_ARGS_REQ(1));
  mrb_define_method(mrb, mod, "define_method",           mod_define_method,        MRB_ARGS_ARG(1,1));
  mrb_define_method(mrb, mod, "class_variables",         mrb_mod_class_variables,  MRB_ARGS_NONE()); /* 15.2.2.4.19 */
  mrb_define_method(mrb, mod, "===",                     mrb_mod_eqq,              MRB_ARGS_REQ(1));
  mrb_define_class_method(mrb, mod, "constants",         mrb_mod_s_constants,      MRB_ARGS_ANY());  /* 15.2.2.3.1 */
  mrb_define_class_method(mrb, mod, "nesting",           mrb_mod_s_nesting,        MRB_ARGS_REQ(0)); /* 15.2.2.3.2 */

  mrb_undef_method(mrb, cls, "append_features");
  mrb_undef_method(mrb, cls, "extend_object");

  mrb->top_self = (struct RObject*)mrb_obj_alloc(mrb, MRB_TT_OBJECT, mrb->object_class);
  mrb_define_singleton_method(mrb, mrb->top_self, "inspect", inspect_main, MRB_ARGS_NONE());
  mrb_define_singleton_method(mrb, mrb->top_self, "to_s", inspect_main, MRB_ARGS_NONE());
  mrb_define_singleton_method(mrb, mrb->top_self, "define_method", top_define_method, MRB_ARGS_ARG(1,1));
}

The product dereferences a pointer that it expects to be valid but is NULL.