diff --git a/phono3py/phonon/grid.py b/phono3py/phonon/grid.py
index c34bcc9f..5997de66 100644
--- a/phono3py/phonon/grid.py
+++ b/phono3py/phonon/grid.py
@@ -684,6 +684,7 @@ def _set_mesh_numbers(
                     self._D_diag = length2mesh(
                         length, self._lattice, rotations=symmetry_dataset.rotations
                     )
+                self._D_diag = np.array(self._D_diag, dtype="long")
         if num_values == 9:
             self._run_grg(
                 symmetry_dataset,