I expect you're bumping into a Django 1.3 bug #15785
. (Also discussed on the mailing list, here
One of the aspects of that bug is that `request.read(9999999)` will raise an assertion error when used against the test client,
rather than simply returning all the remaining stream. It's fixed in 1.4. (Randomly, that was my first contribution to core Django.)
Another way for this error to occur might be if you're overriding the test client and instantiating the dummy request incorrectly.
I guess it's one of those two things.