How to use the pre_get_comments action to alter comment meta query vars

pre-get-comments.php

<?php

function cws_pre_get_comments( $comments ) {
	$comments->query_vars['meta_key'] = 'foo';
	$comments->query_vars['meta_value'] = 'bar';
	// Because at this point, the meta query has already been parsed,
	// we need to re-parse it to incorporate our changes
	$comments->meta_query->parse_query_vars( $comments->query_vars );
}

add_action( 'pre_get_comments', 'cws_pre_get_comments' );

I think you need to do:

 $comments->meta_query = new WP_Meta_Query();

before doing

 $comments->meta_query->parse_query_vars( $comments->query_vars );

it works great without it