NS Flex negative balance

Good morning!
About 4 months ago there was a power down across the NS services and all the metro and train station gates were open, so I entered and exited without using my card. However, when travelling home from work, I entered the station through the open gates, but when I arrived at my home station the gates were closed, and so I had to use my card to exit. Since then, there has been this -€4 balance note in my NS account, and I don’t know how to fix it:

 I presumed it would be paid for in my next invoice, but that hasn’t happened.

Can anybody lend any advice as to why this happened, and how I can pay this fee? Many thanks in advance!