get_posts with excluding category

Hi all.

The version of my WordPress is 3.5.1

I modified my front-page file.
This code gets the most recent post from the loop.
I want to exclude a category.
The id of the category is ‘6’.

<?php
$args = array( 'numberposts' => 1, 'exclude' => '6' );
$lastposts = get_posts( $args );
foreach($lastposts as $post) : setup_postdata($post); ?>
	<h2><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
	<?php the_content(); ?>
<?php endforeach; ?>

I also tried
$args = array( 'numberposts' => 1, 'cat => -6' );

But it does not work: in the home page show all, including the post of the excluded category.

I read the documentation and this old topic, but these pages didn’t help me

I wonder where I am going wrong.

Thank you