python照书上例子输入的，为什么运行时出错，对照了几次没输错地方啊。哪位大神帮我看看

class Dog():
    """一次模拟小的简单尝试"""
 
    def _init_(self, name, age):
        """初始化属性name和age"""
        self.name = name
        self.age = age
 
    def sit(self):
        """模拟小狗被命令时蹲下"""
        print(self.name.title() + " is now sitting.")
 
    def roll_over(self):
        """模拟小狗被命令时打滚"""
        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.")

运行后提示错误：
Traceback (most recent call last):
  File "F:\python\9\dog.py", line 17, in <module>
    my_dog = Dog('willie', 6)
TypeError: object() takes no parameters