I discovered this issue with Kubuntu 14.04 which I installed from scratch using a CD/DVD. During installation I selected full-disk encryption as well as an encrypted home directory -- my understanding is that home directory encryption also encrypts the swap space.

Below is the current state of the drives. Note that:
1. 'blkid' does not list swap, and /proc/swaps is empty; however,
2. /etc/crypttab has entries that apparentlly correspond to the encrypted disk as well as the swap, as does /etc/fstab; but
3. 'swapon' does not like the swap drive/s.

I only have a single physical disk (plus the optical drive).


======blkid===============
$ sudo blkid
/dev/sda1: UUID="f8ac0867-47e7-4c5c-8988-ab680388df78" TYPE="ext2" 
/dev/sda5: UUID="2fb151ab-a2e3-4007-9cff-a61034e7a0c7" TYPE="crypto_LUKS" 
/dev/mapper/sda5_crypt: UUID="8tXqxi-M5bU-r75F-4VPY-ur6s-60P6-3qK3uV" TYPE="LVM2_member" 
/dev/mapper/kubuntu--vg-root: UUID="67d318ec-2121-4da9-a705-7258033d64b3" TYPE="ext4"


=========fdisk===========
$ sudo fdisk -l

Disk /dev/sda: 320.1 GB, 320072933376 bytes
255 heads, 63 sectors/track, 38913 cylinders, total 625142448 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00037417

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      499711      248832   83  Linux
/dev/sda2          501758   625141759   312320001    5  Extended
/dev/sda5          501760   625141759   312320000   83  Linux

Disk /dev/mapper/sda5_crypt: 319.8 GB, 319813582848 bytes
255 heads, 63 sectors/track, 38881 cylinders, total 624635904 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/mapper/sda5_crypt doesn't contain a valid partition table

Disk /dev/mapper/kubuntu--vg-root: 313.4 GB, 313377423360 bytes
255 heads, 63 sectors/track, 38099 cylinders, total 612065280 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/mapper/kubuntu--vg-root doesn't contain a valid partition table

Disk /dev/mapper/kubuntu--vg-swap_1: 6417 MB, 6417285120 bytes
255 heads, 63 sectors/track, 780 cylinders, total 12533760 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/mapper/kubuntu--vg-swap_1 doesn't contain a valid partition table


=========/etc/fstab==========
# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
/dev/mapper/kubuntu--vg-root /               ext4    errors=remount-ro 0       1
# /boot was on /dev/sda1 during installation
UUID=f8ac0867-47e7-4c5c-8988-ab680388df78 /boot           ext2    defaults        0       2
/dev/mapper/kubuntu--vg-swap_1 none            swap    sw              0       0
/dev/mapper/cryptswap1 none swap sw 0 0


=======/etc/crypttab==========
sda5_crypt UUID=2fb151ab-a2e3-4007-9cff-a61034e7a0c7 none luks,discard
cryptswap1 UUID=5f846409-3a3c-43fa-8db0-88e41daf620a /dev/urandom swap,cipher=aes-cbc-essiv:sha256


=======swapon -a===========
$ sudo swapon -a
swapon: /dev/mapper/kubuntu--vg-swap_1: read swap header failed: Invalid argument
swapon: /dev/mapper/cryptswap1: stat failed: No such file or directory


=======/proc/swaps==========
$ cat /proc/swaps
Filename                                Type            Size    Used    Priority