Simple custom post query for all the posts or many of them not working

I am doing this query

$args = array(
'posts_per_page' => '-1',
'post_type' => 'custom_post_type',
'post_status' => 'publish'
);

query_posts( $args );

but it is not working

I have around 800 posts in that custom type and the query_posts function is working just if posts_per_page < 519. I know is weird but if I do this

$args = array(
'posts_per_page' => '518', //or any lower number
'post_type' => 'custom_post_type',
'post_status' => 'publish'
);

query_posts( $args );

it does work but not with

$args = array(
'posts_per_page' => '519', //or bigger
'post_type' => 'custom_post_type',
'post_status' => 'publish'
);

query_posts( $args );

Any suggestion???

Thanks