wubi 12.04 fails to run due to unrelated iso in partition root

[Nils Maier]

wubi 12.04 fails to run, i.e. aborts early before showing the UI.
On the very same system wubi 11.04 runs, so this is a regression.

The culprit was an unrelated iso image that the 7z cannot open. In my case I had E:\snowleo.iso, which OSX SnowLeopard. E: is a regular ntfs hard disk partition on locally attached storage (non-USB). After failing to open that iso the installer just quits.

tail of wubi-12.04-rev266.log:
06-14 13:58 DEBUG CommonBackend: Searching ISOs on USB devices
06-14 13:58 DEBUG Distro: checking Ubuntu ISO E:\snowleo.iso
06-14 13:58 ERROR WindowsBackend: Error executing command
>>command=C:\Users\bootstrap\Downloads\wubi12\bin\7z.exe l E:\snowleo.iso
>>retval=2
>>stderr=

7-Zip 9.20 Copyright (c) 1999-2010 Igor Pavlov 2010-11-18

Error: E:\snowleo.iso: Can not open file as archive

Errors: 1
>>stdout=
Traceback (most recent call last):
  File "\lib\wubi\backends\win32\backend.py", line 519, in get_iso_file_names
  File "\lib\wubi\backends\common\utils.py", line 66, in run_command
Exception: Error executing command
>>command=C:\Users\bootstrap\Downloads\wubi12\bin\7z.exe l E:\snowleo.iso
>>retval=2
>>stderr=

7-Zip 9.20 Copyright (c) 1999-2010 Igor Pavlov 2010-11-18

Error: E:\snowleo.iso: Can not open file as archive

Errors: 1
>>stdout=
06-14 13:58 DEBUG WindowsBackend: command >>C:\Users\bootstrap\Downloads\wubi12\bin\7z.exe l E:\snowleo.iso
06-14 13:58 DEBUG Distro: does not contain casper\filesystem.squashfs
06-14 13:58 DEBUG Distro: checking Ubuntu ISO E:\snowleo.iso
06-14 13:58 ERROR root: iteration over non-sequence
Traceback (most recent call last):
  File "\lib\wubi\application.py", line 57, in run
  File "\lib\wubi\backends\common\backend.py", line 190, in fetch_basic_info
  File "\lib\wubi\backends\common\backend.py", line 803, in find_any_iso
  File "\lib\wubi\backends\common\distro.py", line 113, in is_valid_iso
TypeError: iteration over non-sequence