Error Message - need help

Traceback (most recent call last):
File "site-packages\calibre\utils\ipc\simple_worker.py", line 284, in main
File "site-packages\calibre\ebooks\oeb\iterator\book.py", line 64, in extract_book
File "site-packages\calibre\customize\conversion.py", line 245, in __call__
File "site-packages\calibre\ebooks\conversion\plugins\html_in put.py", line 87, in convert
File "site-packages\calibre\ebooks\conversion\plumber.py", line 1292, in create_oebbook
File "site-packages\calibre\ebooks\oeb\reader.py", line 72, in __call__
File "site-packages\calibre\ebooks\oeb\reader.py", line 703, in _all_from_opf
File "site-packages\calibre\ebooks\oeb\reader.py", line 348, in _spine_from_opf
OEBError: Spine is empty


I am trying to convert to mobi but I keep getting this message.
Can anyone please help me?
It's my first time using Calibre and I really like the software, but this error message is doing my head in.

Thanks in advancce.