2.6.18 netlink_kernel_create() always returns NULL

[YANG]

My code fragment is below:

"nl_sk = netlink_kernel_create(16, 0, nl_data_ready, THIS_MODULE);"

It can be "make" and "insmod" correctly. But error mesage is printed
into log. It indicates the netlink_kernel_create() returns NULL.

Once I change the firt parameter to another(<32 && >16), system crash
when I execute insmod.

Is the protocol ID occupied? Why can't use another and how to define a
new ID? I have tried to update net_link.h but fail also.