导致分段错误的PyArg\u ParseTuple

#import "Python.h"

...

// Called from python with test_method(0, 0, 'TEST')
static PyObject*
test_method(PyObject *args)
{
    int ok, x, y, size;
    const char *s;

    // this causes Segmentation fault
    //ok = PyArg_ParseTuple(args, "iis#", &x, &y, &s, &size); 

    // also segfaults
    //if(ok) PyErr_SetString(PyExc_SystemError, 'Exception');

    // this does not cause segfault but fills the variables with garbage   
    ok = PyArg_ParseTuple(&args, "iis#", &x, &y, &s, &size);

    // Example: >test_method 0, 37567920, (garbage)
    printf(">test_method %d, %d, %s\n", x, y, s);

    /* Success */
    Py_RETURN_NONE;
}

static PyMethodDef testMethods[] =
{
     {"test_method", test_method, METH_VARARGS,
             "test_method"},
     ...

     {NULL, NULL, 0, NULL}
};

知道我做错了什么吗(Python版本2.6.4)。