Anaconda crashes during partitioning with the error "TypeError: cannot concatenate 'str' and 'NoneType' objects", why ?

Issue

• Anaconda crashes while installing the machine with the following partitioning details.

    zerombr
    clearpart --all --initlabel
    bootloader --driveorder=cciss/c0d0,sda

    part /boot  --size=250 --onbiosdisk=80
    part / --size=1 --grow --onbiosdisk=80

• Anaconda crashes with an un-handled exception.

    self.handler.handlers[cmd](cmdArgs)
    File "/usr/lib/anaconda/kickstart.py", line 530, in doPartition
    if not hds.has_key(pd.disk) and hds.has_key('mapper/'+pd.disk):
    TypeError: cannot concatenate 'str' and 'NoneType' objects
    install exited abnormally [1/1]
    sending termination signals...done