Incorrect vertex normals when loading obj file

[NarinderS]

I'm trying to load a very simple obj file:

o Plane
v 0.000000 0.000000 0.000000
v 0.353553 0.353553 0.000000
v 0.353553 -0.353553 0.000000
vn -0.0000 -0.0000 1.0000
vt 0.500000 0.250000
vt 0.500000 0.500000
vt 0.250000 0.500000
s 0
f 2/1/1 1/2/1 3/3/1

When I load the vertex normals using the function load_mesh_vn, I get normals that look like:

[[0 0 1]
 [0 0 0]
 [0 0 0]]

when I would expect something like this instead:

[[0 0 1]
 [0 0 1]
 [0 0 1]]

[NarinderS]

With a slightly more complex obj file:

o Plane
v 0.000000 0.000000 0.000000
v 0.353553 0.353553 0.000000
v 0.353553 -0.353553 0.000000
v 0.000000 -0.707107 -0.000000
v -0.000000 0.707107 0.000000
vn -0.0000 -0.0000 1.0000
vt 0.500000 0.500000
vt 0.250000 0.750000
vt 0.250000 0.500000
vt 0.500000 0.250000
vt 0.750000 0.250000
s 0
f 1/1/1 4/2/1 3/3/1
f 2/4/1 1/1/1 3/3/1
f 5/5/1 1/1/1 2/4/1

I've noticed that sometimes I'll get a nan in the normal matrix,

[[-0.00000000e+00 -0.00000000e+00  1.00000000e+00]
 [ 0.00000000e+00  4.48415509e-44  0.00000000e+00]
 [ 8.53390765e-43  0.00000000e+00  2.80259693e-45]
 [ 0.00000000e+00 -1.03065395e+09  4.02691140e-41]
 [ 7.60905066e-43  0.00000000e+00             nan]]