KeyError: u'MimeType'

[Geremia]

I get this error when trying to open a DjVu or PDF file in Calibre (by typing "o"):

calibre, version 2.78.0
ERROR: Unhandled exception: <b>KeyError</b>:u'MimeType'

calibre 2.78  embedded-python: True is64bit: True
Linux-4.4.38-x86_64-with-slackware-14.2 Linux ('64bit', 'ELF')
('Linux', '4.4.38', '#2 SMP Sun Dec 11 16:11:02 CST 2016')
Python 2.7.9
Linux: ('Slackware ', '14.2', '')
Traceback (most recent call last):
  File "site-packages/calibre/gui2/dialogs/choose_format.py", line 76, in choose_open_with
  File "site-packages/calibre/gui2/open_with.py", line 313, in choose_program
  File "site-packages/calibre/gui2/open_with.py", line 220, in finalize_entry
KeyError: u'MimeType'

For some reason, it seems to think Okular is not a DjVu or PDF reader, yet it is set as one in my system's file associations. Is this a Calibre issue or something on my end?

Update: I just discovered it was an associations issue on my end.

[theducks]

for reporting back

[kovidgoyal]

I'm confused, at least with default keybindings, typing O opens the folder that contains the book file. Have you re-mapped o to something else?

[Geremia]

Originally Posted by kovidgoyal

I'm confused, at least with default keybindings, typing O opens the folder that contains the book file. Have you re-mapped o to something else?

I got the same problem when typing "v" to open it in a viewer and when typing "o" to open the containing folder. For some reason, my system's file associations caused "v" to open the book in my web browser and "o" to open the containing folder also in my web browser. But correcting my system's file associations fixed the Calibre error.