BuildKernel Error

[Sila Sujjinanont]

Help me please.
i make bulidkernel see this error.
cc -shared -nostdlib hack.c -o hack.So
rm -f hack.c
sh /usr/src/sys/conf/newvers.sh GENERIC
cc -c -O -pipe -mtune=pentiumpro -Wall -Wredundant-decls -Wnested-externs
-Wstrict-prototypes  -Wmissing-prototypes -Wpointer-arith -Winline
-Wcast-qual  -ansi -g -nostdinc -I. -I/usr/src/sys
-I/usr/src/sys/../include -I/usr/src/sys/contrib/dev/acpica
-I/usr/src/sys/contrib/ipfilter  -D_KERNEL -include opt_global.h
-mpreferred-stack-boundary=2 -fno-common -ffreestanding  vers.c
linking kernel.debug
if.o(.text+0x47d): In function `if_detach':
/usr/src/sys/net/if.c:322: undefined reference to `ether_poll_deregister'
*** Error code 1
Stop in /usr/obj/usr/src/sys/dksh.
*** Error code 1
Stop in /usr/src.
*** Error code 1
Stop in /usr/src.

Thank.