array_slice confused about negative length

Why is the second array below empty? It seems array_splice cannot grok negative length, despite its docs.

<?php
$array
= array('a' => '1');

// GOOD
print_r(array_slice($array, 010));

// WTF. Why empty array?
print_r(array_slice($array, 0,  -10));
?>

Array
(
    [a] => 1
)
Array
(
)