Wordpress插件的基本设置问题

我一直试图将设置保存到我正在创建的插件的Wordpress数据库中，但我一直被抛出

"警告：中的字符串偏移量'iconomysignup_amount'非法/home/tomatocr/public_html/wp-content/plugins/iconomysignup/iconomy-signup.php在线52"

以下是我正在使用的插件代码：

<?php
    /**
     * Plugin Name: Tomatocraft iConomy Signup
     * Description: A simple plugin that gives a user-defined amount of coins to the user who just signed up.
     * Version: 1.0
     * Author: minejames1
     * Author URI: http://twitter.com/jaycraft_
     */
    // add the admin options page
    add_action('admin_menu', 'plugin_admin_add_page');
    function plugin_admin_add_page() {
        add_options_page('Tomatocraft iConomy Signup', 'iConomy Signup', 'manage_options', 'iconomysignup', 'plugin_options_page');
    }
    // display the admin options page
    function plugin_options_page() {
?>
<div class="wrap">
    <?php screen_icon();?>
    <h2>Set signup iConomy reward</h2>
    <form action="options.php" method="post">
        <?php settings_fields('plugin_options'); ?>
        <?php do_settings_sections('iconomysignup'); ?>
        <?php submit_button(); ?>
    </form>
</div>
<?php
    } // end of page
    // add the admin settings and such
    add_action('admin_init', 'plugin_admin_init');
    function plugin_admin_init(){
        register_setting( 'plugin_options', 'plugin_options', 'plugin_options_validate' );
        add_settings_section('plugin_main', 'Main Settings', 'plugin_section_text', 'iconomysignup');
        add_settings_field('plugin_iconomysignup_amount', 'Amount of Tomatoes', 'plugin_setting_string', 'iconomysignup', 'plugin_main');
    }
    function plugin_options_validate($input) {
        $options = get_option('plugin_options');
        $options['iconomysignup_amount'] = trim($input['iconomysignup_amount']);
        if(!is_numeric($options['iconomysignup_amount'])) {
            $options['iconomysignup_amount'] = '';
        }
        return $options;
    }
    function plugin_section_text() {
        echo '<p>Enter in the amount of Tomatoes the user should receive when signing up.</p>';
    }
    function plugin_setting_string() {
        $options = get_option('plugin_options');
        echo "<input id='plugin_iconomysignup_amount' name='plugin_options[iconomysignup_amount]' size='40' type='text' value='{$options['iconomysignup_amount']}' />";
    }
?>

如果有人能指出我犯的错误，那就太好了——这些错误可能很愚蠢，也很明显，但作为Wordpress CMS的初学者，这一切都很令人困惑。谢谢