NULL pointer deference Oops: 0000 in 2.1.6[89]

Immediately after seeing:  INIT: Entering runlevel: 3 on boot-up
I've been locking up with:
  Unable to handle kernel NULL pointer dereference at virtual
  address 00000004  current->tss_cr3 = 04d68000, %cr3 = 04d68000
  *pde = 00000000 Oops: 0000

I've been compiling (2.1.69) with CONFIG_M586=y (on an older Pentium)
with SMP commented out in the Makefile (why is SMP the default?) and
no modules (only =y)

Are there any hints anyone might care to offer?
Many thanks to you all.
-- 
jeff [e-mail]