Calibre error "Cannot operate on null QImage"

It was working fine till i tried adding a cover which was not a valid jpg file
and since then it is giving me error
i get the following error when i click the "generate cover" icon

ValueError: Cannot operate on null QImage

please help me get back the cover of my choice on the books.

i get the following text on error

calibre 2.84 embedded-python: True is64bit: False
Windows-7-6.1.7600 Windows ('32bit', 'WindowsPE')
('Windows', '7', '6.1.7600')
Python 2.7.9
Windows: ('7', '6.1.7600', '', 'Multiprocessor Free')
Successfully initialized third party plugins: K4PC, K4Mac, Kindle Mobi and Topaz DeDRM (0, 3, 7) && Inept Epub DeDRM (0, 1, 7) && Inept PDF DeDRM (0, 1, 4) && Generate Cover (1, 5, 21) && Ignoble Epub DeDRM (0, 1, 6) && eReader PDB 2 PML (0, 0, 5)
Traceback (most recent call last):
File "calibre_plugins.generate_cover.action", line 65, in _generate_cover
File "calibre_plugins.generate_cover.dialogs", line 1377, in __init__
File "calibre_plugins.generate_cover.dialogs", line 1669, in display_preview
File "calibre_plugins.generate_cover.draw", line 330, in generate_cover_for_book
File "calibre_plugins.generate_cover.draw", line 178, in create_cover_page
File "site-packages\calibre\utils\magick\legacy.py", line 117, in compose
File "site-packages\calibre\utils\img.py", line 246, in overlay_image
ValueError: Cannot operate on null QImage


P.S i guess its a cover generator plugin error.