Problem with wp_create_user

Please be a little patient as I’m fairly new to wordpress architecture. I’m having an issue with the wp_create_user() function. I’m trying to create a custom registration form page for my site.

This code is what should execute after submit:

<?php
	include('./wp-blog-header.php');

	$theUser = $_POST['theUser'];
	$theEmail = $_POST['theEmail'];
	$random_password = generate_random_password(12, false, false);

	$user_id = username_exists( $theUser );

	if ( !$user_id ) {
		$user_id = wp_create_user( $theUser, $random_password, $theEmail );
	} else {
		$random_password = __('User already exists.  Password inherited.');
	}
}

Most of the code was copied from the wp_create_user() function reference page.

I get this error:

Fatal error: call to member function get_row() on a non-object in wp-includes/capabilities.php on line 530

Removing the whole username_exist code creates this error:

Fatal error: call to member function escape() on a non-object in wp-includes/formatting.php on line 2295

I’m not sure what my error is. Everything I find says there’s a problem with my $wpdb but whether I use global $wpdb in any area of the code above it still doesn’t work.

Any help is greatly appreciated. Thanks.