"coverage combine" consumes a lot of memory
When trying to use "coverage combine" in my circleci.com CI build  I get a message saying: "Warning: The build VMs have a memory limit of 4GB. Your build hit this limit on one or more containers, and your build results are likely invalid."
Removing the "coverage combine" command from the build fixes the build failure, so I'm pretty confident that this is a bug in coverage.py and not in circle.com or anything else.
The software I'm trying to measure code coverage on is w3af , which rather large (lots of python files, lots of lines).
Coverage information is gathered using nosetests with "--with-cov --cov-report=xml".
Any ideas on how to fix this?