Undefined variable: post

I display the current taxonomy name above the product title. I added this code to title.php file :

the_terms( $post->ID, 'xxx', '<span>', ' , ', '</span>' );

It works fine, however when I switch debugging on I see this error:
Notice: Undefined variable: post in …/…/title.php

Why is it appearing?

https://ww.wp.xz.cn/plugins/woocommerce/