Default sort order of invoices

Hi,

I use different prefixes for the invoice numbers depending on customer / customer types.

This, unfortunately, is not working very well with the default sorting (invoice number, desc) of the invoice list (/index.php/incomes/invoices).
I need to click two times on the invoice date colum to get the list sorted descending by invoice date.

To me, this would be a more reasonable default.
Can you point me in the right direction, where to change this in the code?
I dug around a bit, but wasn't able to find it... :/

Thanks!

You can check app/Http/Controllers/Incomes/Invoices.php file.