Extending WP_Widget throws error when Monster Widget used

I wanted to override the output for the core Recent Comments Widget. I extended WP_Widget_Recent_Comments made the changes needed and it displays exactly how I want. However, I have now tested the theme with Monster widget, and for where the Recent Comments widget should be output an error is thrown:

Notice: Undefined index: WP_Widget_Recent_Comments in /Users/tfisher/Sites/mysite/dev/wp-includes/widgets.php on line 1319

Can’t figure out why it would only throw an error when used by Monster Widget.

Here’s my extension class:

Class My_Recent_Comments_Widget extends WP_Widget_Recent_Comments {

    function widget( $args, $instance ) {
        global $comments, $comment;

        $cache = wp_cache_get('widget_recent_comments', 'widget');

        if ( ! is_array( $cache ) )
            $cache = array();

        if ( ! isset( $args['widget_id'] ) )
            $args['widget_id'] = $this->id;

        if ( isset( $cache[ $args['widget_id'] ] ) ) {
            echo $cache[ $args['widget_id'] ];
            return;
        }

        extract($args, EXTR_SKIP);
        $output = '';

        $title = ( ! empty( $instance['title'] ) && isset($instance['title']) ) ? $instance['title'] : __( 'Recent Comments', 'mysite' );

        /** This filter is documented in wp-includes/default-widgets.php */
        $title = apply_filters( 'widget_title', $title, $instance, $this->id_base );

        $number = ( ! empty( $instance['number'] ) && isset($instance['number']) ) ? absint( $instance['number'] ) : 5;
        if ( ! $number )
            $number = 5;

        /**
         * Filter the arguments for the Recent Comments widget.
         *
         * @since 3.4.0
         *
         * @see get_comments()
         *
         * @param array $comment_args An array of arguments used to retrieve the recent comments.
         */
        $comments = get_comments( apply_filters( 'widget_comments_args', array(
            'number'      => $number,
            'status'      => 'approve',
            'post_status' => 'publish'
        ) ) );

        $output .= $before_widget;
        if ( $title )
            $output .= $before_title . $title . $after_title;

        if ( $comments ) {
            // Prime cache for associated posts. (Prime post term cache if we need it for permalinks.)
            $post_ids = array_unique( wp_list_pluck( $comments, 'comment_post_ID' ) );
            _prime_post_caches( $post_ids, strpos( get_option( 'permalink_structure' ), '%category%' ), false );

            foreach ( (array) $comments as $comment) {
                $output .= '<div class="recentcomments item clearfix">';
                if(get_avatar($comment)) {
                    $output .= '<figure class="pull-left">';
                    $output .= '<a href="' . get_comment_author_url() . '">';
                    $output .= get_avatar( $comment, 100 );
                    $output .= '</a>';
                    $output .= '</figure>';
                    $output .= '<div class="content">';
                } else {
                    $output .= '<div class="content no-img">';
                }
                $output .= sprintf(_x('%1$s on %2$s', 'widgets'), get_comment_author_link(), '<a href="' . esc_url( get_comment_link($comment->comment_ID) )) . '">';
                $output .= get_the_title($comment->comment_post_ID) . '</a>';
                $output .= '</div>'; // content
                $output .= '</div>'; // item
            }
        }
        $output .= $after_widget;

        echo $output;
        $cache[$args['widget_id']] = $output;
        wp_cache_set('widget_recent_comments', $cache, 'widget');
    }
}