Error Calling a member function row () on a non-object Codeigniter

Password change screen, form is ok, but the variable to fetch the information in the bd giving error, in the row () function!

Follow VIEW:

case 'alterar_senha':
        $iduser= $this->uri->segment(3);
        if($iduser=NULL):
          set_msg('msgerro', 'Escolha um usuário para alterar', 'erro');
          redirect('usuarios/gerenciar');
        endif; ?>
        <div class="large-12 columns">
            <?php if (is_admin() || $iduser == $this->session->userdata('user_id')):
                $query = $this->usuarios->get_byid($iduser)->row();
                erros_validacao();
                get_msg('msgok');
                echo form_open(current_url(), array('class'=>'custom'));
                echo form_fieldset('Alterar Senha');
                echo '<div class="large-7 columns">';
                echo form_label('Nome completo');
                echo form_input(array('name'=>'nome', 'disabled'=>'disabled'), set_value('nome', $query->nome));
                echo form_label('Email');
                echo form_input(array('name'=>'email', 'disabled'=>'disabled'), set_value('email', $query->email));
                echo '<div class="large-7 columns>"';
                echo form_label('Login');
                echo form_input(array('name'=>'login',  'disabled'=>'disabled'), set_value('login', $query->login));
                echo form_label('Nova Senha');
                echo form_password(array('name'=>'senha'), set_value('senha'), 'autofocus');
                echo form_label('Repita a senha');
                echo form_password(array('name'=>'senha2'), set_value('senha2'));
                echo anchor('usuarios/gerenciar', 'Cancelar', array('class'=> 'button radius alert espaco'));
                echo form_submit(array('name'=>'alterarsenha', 'class'=>'button radius '), 'Salvar dados');
                echo form_hidden('idusuario', $iduser);
                echo form_fieldset_close();
                echo form_close();
            else:
                redirect('usuarios/gerenciar');
            endif;  ?>
        </div>

        <?php
        break;

Follow CONTROLLER with function:

public function alterar_senha(){
    esta_logado();
    set_tema('titulo','Alteração de senha');
    set_tema('conteudo',load_modulo('usuarios','alterar_senha'));
    load_template();
}

Follow MODEL:

public function get_byid($id=NULL) {
        if ($id != NULL):
            $this->db->where('id', $id);
            $this->db->limit(1);
            return $this->db->get('usuarios');
         else:
            return FALSE;   
         endif;
    }