error in make phase while compiling kernel

[ashadul hoque]

Hi everyone

I am observing the following error messages of the following type while 
compiling custom kernel.

umass.o: In function 'umass_cam_detach_sim':
umass.o(.test+0x1a5c): undefined reference to 'xpt_bus_deregister'
umass.o(.test+0x1a78): undefined reference to 'cam_sim_free'

What is the reason?

regards
Ashadul