How can I use "post__not_in" with pre_get_posts?

Hello, I checked many examples but couldn’t get this working.
Here is the code I’m using:

function exclude_some_posts( $query ) {
	if ( $query->is_main_query() ) {
	   $query->set('post__not_in', array(1));
	}}
add_action( 'pre_get_posts', 'exclude_some_posts' );