Expose custom taxonomies in CSV impot / export

Hello,
I tried to add a code my theme functions to expose custom taxonomies (made with CPT UI) in CSV import / export. Based on your code at https://github.com/woocommerce/woocommerce/wiki/Product-CSV-Importer-&-Exporter#adding-custom-export-columns-developers I made this:

function add_export_column( $columns ) {

	if (is_admin()) {

		// column slug => column name
		$columns['book_author'] = 'Author(s)';
		$columns['publisher'] = 'Publisher';
		$columns['series'] = 'Book series';

		return $columns;
	}
}
add_filter( 'woocommerce_product_export_column_names', 'add_export_column' );
add_filter( 'woocommerce_product_export_product_default_columns', 'add_export_column' );

function add_export_data_book_author( $value, $product ) {
	if (is_admin()) {
		$value = $product->get_the_terms( 'book_author' );
		return $value;
	}
}
add_filter( 'woocommerce_product_export_product_column_book_author', 'add_export_data_book_author', 10, 2 );

function add_export_data_publisher( $value, $product ) {
	if (is_admin()) {
		$value = $product->get_the_terms( 'publisher' );
		return $value;
	}
}
add_filter( 'woocommerce_product_export_product_column_publisher', 'add_export_data_publisher', 10, 2 );

function add_export_data_series( $value, $product ) {
	if (is_admin()) {
		$value = $product->get_the_terms( 'series' );
		return $value;
	}
}
add_filter( 'woocommerce_product_export_product_column_series', 'add_export_data_series', 10, 2 );

Now my taxonomies are exposed in export configuration, but if I include them into export config I got only endless spinner after pressing Generate button. Can’t find my mistake, hope you’ll help.