FreeBSD make: don't know how to make buildkernel.

Q. I'm trying to recompile my kernel by tying the following commands:

cd /usr/src/sys/i386/conf
cp GENERIC NEWKEN
vi NEWKEN
cd /usr/src
make buildkernel KERNCONF=NEWKEN

But I'm bumped with an error that read as follows:
make: don't know how to make buildkernel. Stop Error and Solution

How do I fix this error?

A. This is well known problem. You need an up to date and complete copy of the FreeBSD source including kernel at /usr/src and /usr/src/sys. To grab latest FreeBSD kernel source use or sysinstall command.
# sysinstall
Select > Configure > Distributions > Src > Select Base, Sys and all other required source code > Ok > Ok > Select Internet > FTP Main site and install the source code.

Once installed type the following command to compile the kernel:

# cd /usr/src
# make buildkernel KERNCONF=NEWKEN
# make installkernel KERNCONF=NEWKEN
# reboot