大神帮我看看这个问题吧？新手小白刚学习 找不到错误

class Dog():
    def _init_(self,name,age):
        self.name=name
        self.age=age
    def sit(self):
        print(self.name.title()+" is now sitting.")
    def roll_over(self):
        print(self.name.title()+" rolled over!")
my_dog=Dog('willie',6)
print("My dog's name is "+ my_dog.name.title()+".'")
print("My dog  is "+str(my_dog.age)+" years old.")

my_dog=Dog('willie',6)提示这行错误，调用类Dog 不对吗？