Hello! Just looking for a server's perspective on this;;

I ordered a curry from a local restaurant for lunch today. It was sour, like, SUPER sour. At first, I thought they used lemon or some other tangy ingredient as a special type of curry recipe, but about halfway through, I realized it was way too sour to be on purpose. I asked the waitress if the curry was supposed taste sour. She said no and went back into the kitchen to ask the chef where they sampled the curry and she told me it had likely gone bad from the heat (heat wave this weekend). She was extremely apologetic and horrified, and comp'd my meal, and gave me a voucher for a free meal next time. She also told me to call if I felt bad or had any signs of food poisoning.

In my haste (and because there was a line forming), I left and realized I didn't leave a tip and wasn't sure if I was supposed to. I did some googling and it looks like most people agree it's etiquette to pay for the service, regardless of the comp'd meal. Now I feel terrible as it wasn't her fault. Should I go back tomorrow to give her the tip, or just tip extra at my next meal as I'll be back for sure?