Make *id fields unsigned.
function statistics_update_8001() {
db_drop_primary_key('node_counter');
db_change_field('node_counter', 'nid', 'nid', array(
'description' => 'The {node}.nid for these statistics.',
'type' => 'int',
'unsigned' => TRUE,
'not null' => TRUE,
'default' => 0,
), array(
'primary key' => array(
'nid',
),
));
}