Kindle file error (Was:Not sure where to put this sorry!)

I use calibre to convert my amazon kindle files, but there's three that it won't convert using the plugin (can't remember the name at the moment sorry!). This is what comes up:

calibre, version 1.42.0
ERROR: Unhandled exception: <b>IndexError</b>:tuple index out of range

calibre 1.42 [64bit] isfrozen: True is64bit: True
Windows-7-6.1.7601-SP1 Windows ('64bit', 'WindowsPE')
('Windows', '7', '6.1.7601')
Python 2.7.5
Windows: ('7', '6.1.7601', 'SP1', 'Multiprocessor Free')
Traceback (most recent call last):
File "site-packages\calibre\gui2\__init__.py", line 399, in dispatch
File "site-packages\calibre\gui2\actions\convert.py", line 261, in book_converted
File "site-packages\calibre\db\legacy.py", line 277, in add_format
File "site-packages\calibre\db\cache.py", line 1267, in add_format
IndexError: tuple index out of range

Any ideas please?