occasionally, date_add is 0000 when calling $product->save

[Destiny84]

I have built my own import script which fetches product data from a feed and imports them into my prestashop 1.5.4.1

 

When the import is done, about 5% of the products have the following attributes:

date_add : 0000-00-00 00:00:00

minimal_quantity : 0

available_for_order : 0

show_price : 0

 

I don't touch any of these attributes during import so they should have their default values (actual date of import, and 1 for the others). 

 

Does anyone have an idea what might cause this behaviour?