Description of problem:
The error happened when I tried to open anaconda, this to install fedora 21 on an external hard disk drive.

Version-Release number of selected component:
anaconda-core-21.48.6-1.fc21.x86_64

The following was filed automatically by anaconda:
anaconda 21.48.6-1 exception report
Traceback (most recent call first):
  File "/usr/lib/python2.7/site-packages/blivet/udev.py", line 198, in device_get_name
    if "DM_NAME" in udev_info:
  File "/usr/lib/python2.7/site-packages/blivet/devicetree.py", line 1124, in addUdevDevice
    name = udev.device_get_name(info)
  File "/usr/lib/python2.7/site-packages/blivet/devicetree.py", line 713, in addUdevLVDevice
    self.addUdevDevice(pv_info)
  File "/usr/lib/python2.7/site-packages/blivet/devicetree.py", line 1191, in addUdevDevice
    device = self.addUdevLVDevice(info)
  File "/usr/lib/python2.7/site-packages/blivet/devicetree.py", line 2170, in _populate
    self.addUdevDevice(dev)
  File "/usr/lib/python2.7/site-packages/blivet/devicetree.py", line 2105, in populate
    self._populate()
  File "/usr/lib/python2.7/site-packages/blivet/__init__.py", line 469, in reset
    self.devicetree.populate(cleanupOnly=cleanupOnly)
  File "/usr/lib/python2.7/site-packages/blivet/__init__.py", line 188, in storageInitialize
    storage.reset()
  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 227, in run
    threading.Thread.run(self, *args, **kwargs)
TypeError: argument of type 'NoneType' is not iterable

Additional info:
cmdline:        /usr/bin/python  /sbin/anaconda --liveinst --method=livecd:///dev/mapper/live-base
cmdline_file:   BOOT_IMAGE=vmlinuz0 initrd=initrd0.img root=live:CDLABEL=Fedora-Live-Workstation-x86_64-2 rootfstype=auto ro rd.live.image quiet  rhgb rd.luks=0 rd.md=0 rd.dm=0 
executable:     /sbin/anaconda
hashmarkername: anaconda
kernel:         3.16.1-301.fc21.x86_64
other involved packages: python-blivet-0.62-1.fc21.noarch, python-libs-2.7.8-4.1.fc21.x86_64
product:        Fedora
release:        Fedora release 21 (Twenty One)
type:           anaconda
version:        21