Bad handling of zero values

I configured the measured field module to allow me enter a range (from - to) of values. I have written a small module in order to get my own units:

function wine_style_units_define_units() {
  $items = array();
  $items['sweetness'] = array(
    'category_id' => 'sweetness',
    'category' => t('Sweetness'),
    'shortname' => 'g/l',
    'fullname' => 'Sweetness',
    'symbol' => 'g/l',
    'decimals' => 0,
  );

This shows me correctly my unit selector. But after inserting values in the node edit form, no values get inserted into the database table (just the vid and nid though).

Any help would be very much appreciated!

Thank you!