[spinoza]

I've imported a number of public domain .prc files into Calibre, but have been unable to view them. When I try to open them I get the error log below. Can anyone explain what the problem is and how to correct? Many thanks in advance!

unpack requires a string argument of length 8
Traceback (most recent call last):
File "/Applications/ebook and doc manager apps/calibre.app/Contents/console.app/Contents/Resources/lib/python2.6/site-packages.zip/calibre/gui2/viewer/main.py", line 55, in run
Thread.run(self)
File "threading.pyo", line 477, in run
File "calibre/ebooks/epub/iterator.pyo", line 119, in __enter__
File "/Applications/ebook and doc manager apps/calibre.app/Contents/console.app/Contents/Resources/lib/python2.6/site-packages.zip/calibre/ebooks/epub/from_any.py", line 62, in mobi2opf
reader = MobiReader(path)
File "/Applications/ebook and doc manager apps/calibre.app/Contents/console.app/Contents/Resources/lib/python2.6/site-packages.zip/calibre/ebooks/mobi/reader.py", line 258, in __init__
self.book_header = BookHeader(self.sections[0][0], self.ident)
File "/Applications/ebook and doc manager apps/calibre.app/Contents/console.app/Contents/Resources/lib/python2.6/site-packages.zip/calibre/ebooks/mobi/reader.py", line 154, in __init__
self.exth = EXTHHeader(raw[16+self.length:], self.codec, self.title)
File "/Applications/ebook and doc manager apps/calibre.app/Contents/console.app/Contents/Resources/lib/python2.6/site-packages.zip/calibre/ebooks/mobi/reader.py", line 34, in __init__
self.length, self.num_items = struct.unpack('>LL', raw[4:12])
error: unpack requires a string argument of length 8