ppi on 0 balance?

I cleared my cc balance in April and haven’t used my card since. I was charged payment protection in May which I assumed for my balance in April.

I’ve cleared that charge in May so my balance is 0 but I found out I’ve been charged 0.79 for payment protection.

I know it’s only 79p but I thought I shouldn’t be charged anything if my balance is 0?

Thanks.

San