Not at all. You can use any type of squash for this recipe but I usually substitute it with butternut squash or kabocha squash since they have a very similar flavor and sweetness to pumpkins. If you do substitute it with kabocha squash, you will need an extra cup of broth and 1/2 tablespoon of miso to the soup since it has a drier texture than pumpkin. Try not use the large pumpkins that are used for carving on Halloween. Although they are edible, they do not have a lot of flavor to them.