Building example device driver error: insmod invalid module format

I am following the tutorial @ http://www.embeddedsystemonline.com/home/embedded-system/embedded_linux/lab3-linux-device-driver

on step #7 of this tutorial,

$ cd /usr/src/kernels/linux-2.6.20.1/drivers/char/examples$ /sbin/insmod hello1.ko

I then receive the following error (after sudo /sbin/insmod hello1.ko)

insmod: error inserting 'hello1.ko': -1 Invalid module format

Using the lsmod command, I do not see the module in the kernel