add_timer crashes the kernel,Why?

Hello everyone! I want to use the timer in my kernel module. I read the book _Linux kernel development 3rd version_ and LDD 3rd version. Both the two books tell me write the code like this: struct timer_list my_timer; init_timer(&my_timer); my_timer.expires = jiffies + wait; my_timer.data = 0; my_timer.function=my_function; //active the timer: add_timer(&timer); When I insmod the module, the kernel crashed, and when I change my code to *mod_timer* rather than add_timer. It just works! But why the add_timer crash the kernel?