Did you first do 'mkswap' on the partition?

BTW, from what others have said here, it doesn't make a difference anymore if you use a swap partition or a file (particularly with an SD card I should think), because the kernel will initially translate the file to the physical layer so that it sees the same as what it sees in a partition.

BTW, swapon -a will only add swap space which has been defined in /etc/fstab.
swapon /dev/name-of-swap-partition will activate it directly (and it'll last until boot)