Undefined index: user_login in

I’m using this register form on my webpage with your module but get an error when submit:
Undefined index: user_login in /public_html/wp-login.php on line 686

<form action=”<?php echo site_url(‘wp-login.php?action=register’, ‘login_post’) ?>” method=”post”>
<input type=”text” name=”user_email” value=”E-Mail” id=”user_email” class=”input” />
<?php do_action(‘register_form’); ?>
<input type=”submit” value=”Register” id=”register” />

https://www.ads-software.com/plugins/smart-wp-login/