Reproducible on Python 3.6a4ish on Ubuntu. I believe this needs forking multiprocessing.

do_raise is called with 2 NULLs as arguments, it should raise

    PyErr_SetString(PyExc_RuntimeError,
                            "No active exception to reraise");

What happens is that PyThreadState is initialized to *all* NULL pointers on the new thread on multiprocessing, however `type` is expected to point to `Py_None` when no exception has been raised:

        PyThreadState *tstate = PyThreadState_GET();
        PyObject *tb;
        type = tstate->exc_type;
        value = tstate->exc_value;
        tb = tstate->exc_traceback;
        if (type == Py_None) {
            PyErr_SetString(PyExc_RuntimeError,
                            "No active exception to reraise");
            return 0;
        }

I am not sure where the thread state should have been initialized though