No swap file on ZFS?

A developer just requested extra swap space on a Solaris 10 server for debugging purpose. I quickly created a 4GB file using mkfile(1M).

Then, tried to add this space and got the following error:

[ 00:17 root@13-www3 0 ] /zpool # swap -a /zpool/swap
/zpool/swap: Invalid operation for this filesystem type

Oh? /zpool is a ZFS filesystem… If anybody has more info about this… No big deal, I moved the swap file to an UFS volume but I’d like to understand this issue.