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This problem is due to missing Cartridge’s ShopMiddleware. Add this into your settings.py file :
MIDDLEWARE_CLASSES = (
'django.middleware.common.CommonMiddleware',
'django.middleware.csrf.CsrfViewMiddleware',
.......,
.......,
'cartridge.shop.middleware.ShopMiddleware',
)
And if the issue further exists,then compare your settings.py file with This link.
Source:stackexchange.com