Cannot set aggregator teaser length in settings tab.

For whatever reason I haven't found, every settings in /admin/config/services/aggregator/settings works as expected but teaser lenght not.

I mean, when I open the settings page, the option shows the default 600 characters, I set to it to any other value, Save Configuration and when page reload, it fall back to 600 characters.
I tried to delete the aggregator_teaser_length in the variable db table without any success.

I am using Drupal 7 with Nginx + PHP-FPM on a Ubuntu Natty x86.

Value of aggregator_teaser_length is always at:

INSERT INTO `variable` (`name`, `value`) VALUES
('aggregator_teaser_length', 0x733a333a22323030223b);