// callproc.c
static PyObject *format_error(PyObject *self, PyObject *args)
{
    PyObject *result;
    wchar_t *lpMsgBuf;
    DWORD code = 0;
    if (!PyArg_ParseTuple(args, "|i:FormatError", &code))  
                                  ^ Here the format string should be "|I:FormatError"
        return NULL;
    if (code == 0)
        code = GetLastError();
    lpMsgBuf = FormatError(code);
    if (lpMsgBuf) {
        result = PyUnicode_FromWideChar(lpMsgBuf, wcslen(lpMsgBuf));
        LocalFree(lpMsgBuf);
    } else {
        result = PyUnicode_FromString("<no description>");
    }
    return result;
}