Today I had the same error. This was due to a confusing (at least for me) naming scheme.
If you specify your middleware as follows:
'yourapp.SomeMiddleware'
settings.py SomeMiddleware SomeMiddleware.py , . settings.py :
'yourapp.SomeMiddleware.SomeMiddleware'
yourapp/middleware.py django.contrib middlewares, seetings.py :
'yourapp.middleware.SomeMiddleware'