How to convert octree to voxel grid with open3d?

Question

I found to_voxel_grid from open3d documentation, however it didn't show any thing after visualization. I'm wondering if I used the function in the wrong way and how to fix it.

import open3d as o3d
import numpy as np

N = 2000
    
armadillo = o3d.data.ArmadilloMesh()
mesh = o3d.io.read_triangle_mesh(armadillo.path)
pcd = mesh.sample_points_poisson_disk(N)

pcd.scale(1 / np.max(pcd.get_max_bound() - pcd.get_min_bound()), center=pcd.get_center())
pcd.colors = o3d.utility.Vector3dVector(np.random.uniform(0, 1, size=(N, 3)))
    
octree = o3d.geometry.Octree(max_depth=4)
octree.convert_from_point_cloud(pcd, size_expand=0.01)

voxel_grid = octree.to_voxel_grid()
o3d.visualization.draw_geometries([voxel_grid])

This is result when using voxel_grid.get_voxels() checking.

[Voxel with grid_index: (-2147483648, -2147483648, -2147483648), color: (0.361422, 0.375204, 0.305094)]