Description of problem:
The error occurred in the "creating luks on /dev/mapper/fedora-root" step of the Fedora 22 Alpha installation.
I tried to install Fedora 22 Alpha, over a Fedora 21 Workstation installation, which was also LUKS encrypted with a passphrase.
In the start of the installer I chose for "Nederlands" (Dutch) as installer language, but I also experienced this with the English installer.
Upto the Fedora Installation drive detection, I simply follow the installer options without any problems.
When I have to choose a drive to install Fedora 22 Alpha onto, I tick the options to custom partition and to encrypt the harddrive again.
In the custom partitioning I first click on the encrypted harddrive, which I have to open up with my passphrase.
Once opened up, I can see the installer detects the Fedora 21 installation.
I choose to reformat almost all partitions, except the /home partition.
When I accept my partitioning changes, I have to give in the new encryption passphrase, which I choose similar as the old passphrase.
After accepting all changes, the installer starts to partition the new installation, and while busy the installer crashes with this error.

Version-Release number of selected component:
anaconda-core-22.20.2-1.fc22.x86_64

The following was filed automatically by anaconda:
anaconda 22.20.2-1 exception report
Traceback (most recent call first):
  File "/usr/lib/python2.7/site-packages/blivet/formats/__init__.py", line 374, in create
    raise FormatCreateError("invalid device specification", self.device)
  File "/usr/lib/python2.7/site-packages/blivet/formats/luks.py", line 225, in create
    DeviceFormat.create(self, **kwargs)
  File "/usr/lib/python2.7/site-packages/blivet/deviceaction.py", line 577, in execute
    options=self.device.formatArgs)
  File "/usr/lib/python2.7/site-packages/blivet/devicetree.py", line 361, in processActions
    action.execute(callbacks)
  File "/usr/lib/python2.7/site-packages/blivet/__init__.py", line 252, in doIt
    self.devicetree.processActions(callbacks)
  File "/usr/lib/python2.7/site-packages/blivet/osinstall.py", line 1063, in turnOnFilesystems
    storage.doIt(callbacks)
  File "/usr/lib64/python2.7/site-packages/pyanaconda/install.py", line 188, in doInstall
    turnOnFilesystems(storage, mountOnly=flags.flags.dirInstall, callbacks=callbacks_reg)
  File "/usr/lib64/python2.7/threading.py", line 766, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib64/python2.7/site-packages/pyanaconda/threads.py", line 238, in run
    threading.Thread.run(self, *args, **kwargs)
FormatCreateError: ('invalid device specification', '/dev/mapper/fedora-root')

Additional info:
cmdline:        /usr/bin/python2  /sbin/anaconda --liveinst --method=livecd:///dev/mapper/live-base
cmdline_file:   BOOT_IMAGE=/isolinux/vmlinuz0 root=live:LABEL=Fedora-Live-WS-x86_64-22_A-3 ro rd.live.image quiet rhgb
executable:     /sbin/anaconda
hashmarkername: anaconda
kernel:         4.0.0-0.rc1.git0.1.fc22.x86_64
other involved packages: python-blivet-1.0-1.fc22.noarch, python-libs-2.7.9-5.fc22.x86_64
product:        Fedora"
release:        Fedora release 22 (Twenty Two)
type:           anaconda
version:        Fedora