buffer protocol, format_descriptor for long long missing?

buffer = np.array([1,2,3], dtype='int64')

this buffer array is diagnosed by the following code as having format == "l", but the format_descriptor<long long>::format() == q, what's wrong with my implementation then?

// inside pybind c++ function
    auto info = buffer.request();
    if (info.format != py::format_descriptor<long long>::format()) {
      std::cerr << "buffer format is not correct, do nothing, "
          << info.format << " vs " << py::format_descriptor<long long>::format();
    }

Start by reproducing the reported comparison in the shown pybind C++ function using a NumPy int64 buffer, then inspect how buffer.request() and format_descriptor::format() produce their format strings. Done means the mismatch is explained and the expected behavior or required change is identified.