fstab - Why can't I mount a swap partition?

When I try to sudo swapon -a i get this message:

swapon: /dev/sdb4: read swap header failed: Invalid argument

sdb4 is where my swap partition is.

sudo blkid gives back this:

/dev/sdb1: UUID="75622b47-3265-48a6-a697-265b44bdfe55" TYPE="ext4" 
/dev/sdb3: UUID="d638620e-b8b1-49ae-b234-7e89aa0de676" TYPE="ext4" 
/dev/sdb5: UUID="E656-2CAB" TYPE="vfat" 
/dev/mapper/cryptswap1: UUID="83e2e318-eee6-4864-a1c4-330bd16bc6a7" TYPE="swap"

this is the content of fstab:

#                
proc            /proc           proc    nodev,noexec,nosuid 0       0
# / was on /dev/sdb1 during installation
UUID=75622b47-3265-48a6-a697-265b44bdfe55 /               ext4    errors=remount-ro 0       1
# swap was on /dev/sdb4 during installation
#UUID=86d7eebb-b840-4fb0-a1fc-a84074062cb2 none            swap    sw              0       0
#/dev/fd0        /media/floppy0  auto    rw,user,noauto,exec,utf8 0       0   
/dev/sdb4        none            swap    sw              0       0