Form validation: error message

Base URL for registration page:

Code:

http://localhost/codeigniter-project/users/register

Registration page:

PHP Code:

<?php echo validation_errors('<p class="bg-danger">'); ?>
<?php echo form_open('users/register', ['id' => 'register_form', 'class' => 'form-horizontal']); ?>
    <div class="form-group">
        <?php
        echo form_label('First name');
        echo form_input([
            'name' => 'first_name',
            'value' => set_value('first_name'),
            'class' => 'form-control',
            'placeholder' => 'Enter first name',
        ]);
        ?>
    </div>

    ... 


Controller:

PHP Code:

class Users extends CI_Controller
{
    public function register()
    {
        $this->form_validation->set_rules('first_name', 'First name', 'trim|required|min_length[2]');
        $this->form_validation->set_rules('last_name', 'Last name', 'trim|required|min_length[2]');

        if ($this->form_validation->run() == false) {
            return $this->load->view('layouts/main', ['main_view' => 'users/register']);
        }

        ... 


When I open the URL for the very first time there were no validation errors produced.
But if I try to fill the form and for example provide only 'fisrt name' (and do not provide 'last name'), then validation errors generated and displayed on the register page.
So why were no errors genererated in the beginning, when I open the base url for the very first time?