AVR dude - Writing Lock bits for Atmega 8

I'm trying to write lock bits for a new ATmega 8 chip and when i try the following command

avrdude > w lock 0 0xcf

i get an error

avrdude : error writing 0xcf at 0x00000, rc = -6
avrdude : error writing 0xcf at 0x00000 cell 0x0f

what does this mean and how do i overcome this?

I'm using a duemilanove as a programmer with AVRdude..

I've managed to write the bootloader successfully... :wink: