mrb_define_singleton_method(mrb_state *mrb, struct RObject *o, const char *name, mrb_func_t func, mrb_aspec aspec)
{
  prepare_singleton_class(mrb, (struct RBasic*)o);
  mrb_define_method_id(mrb, o->c, mrb_intern_cstr(mrb, name), func, aspec);
}

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