Normal DDBlock with sorting order ascending. More than nine photos goes wrong

Hi,

I have a normal DDBlock with 10 images, named 1,2 etc, 9, 91 (because 10 comes after 1 when sorting ascending).
It goes well with 9 images, but when I add the 10th named 91 it keeps coming before the 9th. So 10th and 9th image are in different order.
May be I am interpreting the sort order wrong. What is the ascending sort order depends on: image names?
Thanks a lot in advance for looking into this!
Greetings, Martijn