Multiple Types Not Working

Although I have set the configure file to allow people to choose multiple blog type, the signup form is only allowing them to choose one type. On the final page of the signup form, I am receiving this error:

Warning: join() [function.join]: Invalid arguments passed in /home/mylinka/public_html/wp-content/mu-plugins/blog-types.php on line 314

Here are lines 309-318 of the code:

//blog subtypes
if ($blog_subtypes_selection == 'single'){
$selected_blog_subtypes = '|' . $_POST['blog_subtype'] . '|';
$selected_blog_types = str_replace("||","|",$selected_blog_types);
} else if ($blog_subtypes_selection == 'multiple') {
$selected_blog_subtypes = '|' . join("|", $_POST['blog_subtypes']) . '|';
$selected_blog_subtypes = str_replace("||","|",$selected_blog_subtypes);
}
$wpdb->query( "INSERT INTO " . $wpdb->base_prefix . "signup_blog_types (blog_types_domain, blog_types_path, blog_types, blog_subtypes) VALUES ( '" . $domain . "', '" . $path . "', '" . $selected_blog_types . "', '" . $selected_blog_subtypes . "' )" );
}