return NotFound () Web Api c #

I need to return a specific message, where it has Get , but it is not working as expected and I tried to implement it separately but it always gives error .

[ResponseType(typeof(pessoa))]
[ActionName("GetCNPJCPF")]
public IHttpActionResult GetCNPJCPF(string cnpj_cpf)
{
     pessoa pessoa = db.pessoa.OfType<pessoa>()
                       .Where(p => p.cpf_cnpj == cnpj_cpf).FirstOrDefault();
     if (pessoa == null)
     {
         return NotFound();
     }
     return Ok(pessoa);
}

If it does not simply find the HTTP 404 error