swap partition question?

[ISHWAR RATTAN <rattan@cps.cmich.edu>]

I have a 1Gb primary swap partition (/dev/sda1)
and fdisk -l shows that. But the command

 # swapon /dev/sda1

says that ..:/dev/sda1: is invalid argument.
I made an entry in /etc/fstab (too):
 /dev/sda1  none  swap sw 0 0

but swapon -a still syays that it is invalid argument!

Any pointers?
-ishwar