PyObject *hook = PyList_GetItem(meta_path, i);
loader = PyObject_CallMethod(hook, "find_module",
  "sO", fullname, path != NULL ? path : Py_None);


The "hook" pointer is checked for NULL in PyObject_CallMethod() -- see
line 1947 in Objects/abstract.c.  The hook is a general object
(hopefully with a find_module() method), not a string.