To help prevent curdling, use high fat dairy products like heavy cream and avoid low fat dairy products like 2% milk.    Next, avoid adding cold dairy products to hot soup. Instead bring the cream to room temperature or gently warm it in a separate pot before adding it to the soup. Tempering is also effective in avoiding curdling.    Do not boil soup after the heavy cream has been added. In fact, I usually recommend turning off heat entirely after the soup is done cooking but before adding the cream.    If you follow the ingredients and instructions in this recipe, there shouldn't be any issues with curdling.