converting iBook

Hi. I just tried to import an iBook file to Calibre to convert it, but Calibre will not open it. I assume this is a DRM issue. What is the best solution?

Here is error log:
calibre, version 2.65.1
WARNING: Errors while adding: There were problems adding some files, click "Show details" for more information


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Failed to add the file /Users/doug/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books/939424053.ibooks to the book: /Users/doug/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books/939424053
With error:
Traceback (most recent call last):
File "site-packages/calibre/gui2/add.py", line 408, in add_formats
File "site-packages/calibre/db/cache.py", line 1374, in add_format
File "site-packages/calibre/startup.py", line 118, in local_open
IOError: [Errno 21] Is a directory: u'/Users/doug/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books/939424053.ibooks'

I am not attaching the iBook file here because it is huge. The ibook includes audio and animations. I understand that these may be lost during conversion, but I'd still like to convert the text.

Many thanks for any advice.