Warning: array_key_exists() on WordPress site. How do I fix? - php

Question

I'm getting the following error you see below on my WordPress site (the weird part is it only appears if a user is logged in)... How can I fix it?
Warning: array_key_exists() [function.array-key-exists]: The first argument should be
either a string or an integer in wp-content/plugins/display-name-author-permalink/display-name-author-permalink.php on line 55
Here is the code:
class DisplayNameAuthorPermaLink {
var $users = array();
// Build an array of usernames and display names and increment duplicates for uniqueness
function __construct() {
$i = 1;
foreach ( get_users() as $user ) {
$display_name = $display_name = sanitize_title($user->display_name);
if ( in_array(sanitize_title($user->display_name), $this->users) ) {
$i++;
$display_name .= "-$i";
}
$this->users[sanitize_title($user->user_login)] = $display_name;
}
add_action('pre_get_posts', array(&$this, 'switch_author'));
add_filter('author_link', array(&$this, 'filter_author'), 10, 3);
}
// Switch the display name with the username so that we can populate the posts properly
// If the username was used in the call do a 404 template redirection
function switch_author() {
if ( ! is_author() )
return;
$author_name = get_query_var('author_name');
$key = array_search($author_name, $this->users);
if ( $key ) {
set_query_var('author_name', $key);
$author = get_user_by('login', $key);
set_query_var('author', $author->ID);
} else {
set_query_var('author_name', false);
set_query_var('author', false);
add_action('template_redirect', array(&$this, 'redirect_404'));
}
}
// Replace the username in author links generated in the theme with the users display name
function filter_author($link,$author_id,$author_nicename) {
if ( array_key_exists($author_nicename, $this->users) )
$link = str_replace($author_nicename,$this->users[$author_nicename], $link);
return $link;
}
// Redirect template to use 404 template
function redirect_404() {
include(get_404_template());
die();
}
}
// Instantiate the DisplayNameAuthorPermaLink class
$DisplayNameAuthorPermaLink = new DisplayNameAuthorPermaLink();