I'm writing a Python wrapper for a C++ library, and I'm getting a really weird when trying to set a struct's field in C. If I have a struct like this:

struct Thing
{
    PyOBJECT_HEAD
    unsigned int val;
};

And have two functions like this:

static PyObject* Thing_GetBit(Thing* self, PyObject* args)
{
    unsigned int mask;

    if(!PyArg_ParseTuple(args, "I", &mask)
        Py_RETURN_FALSE;

   if((self->val & mask) != 0)
       Py_RETURN_TRUE;

    Py_RETURN_FALSE;
}


static PyObject* Thing_SetBit(Thing* self, PyObject* args)
{
    unsigned int mask;
    bool on;

    if(!PyArg_ParseTuple(args, "Ii", &mask, &on))
        Py_RETURN_FALSE;

    if(on)
        thing->val |= mask;
    else
        thing->val &= ~mask;

    Py_RETURN_TRUE;
}

Python code that calls the first method works just fine, giving back the value of the struct member. Calls to the SetBit method give an error about an object at address foo accessing memory at address bar, which couldn't be "written".

I've poked around the code, and it's like I can look at the value all I want, both from C and Python, but the instant I try to set it, it blows up in my face. Am I missing something fundamental here?