I've made this soup twice and it's curdled a bit both times. Any ideas as to how to avoid this?

Instead of just adding the yogurt to the hot soup, temper it first by adding a bit of the warm liquid (a few laddles) to it while whisking, then add that to the pot.

A few ideas: use yogurt with a higher fat content, don't add cold yogurt to hot soup, don't heat it too fast, don't heat it to a boil, and if it does curdle, try pulsing it in a blender.