Swap errors at shutdown

[fcn]

I am sometimes getting swap errors when shutting down. The latest one was:

kernel: swap_pager_getswapspace(1): failed

I've also had swapoff fail.

I have 24 GB of memory and 16 GB of swap which is rarely in use (and for very little space - less than a few MBs). My fstab:

/dev/ada0p3  none  swap  sw  0

swapinfo:

Device  1K-blocks  Used  Avail Capacity
/dev/ada0p3  16777216  0 16777216  0%

This has caused the system going to single user mode on shutdown and killing processes (including a couple of VirtualBox VMs instead of saving their state).

Any info would be appreciated. Thanks.