Zero division error in pack_spheres

Hello everybody,

I am new in OpenMC community. I was trying the example of Modeling TRISO Particle from the Examples section. The error occured calling pack_sphere function:
----> [2] centers = openmc.model.pack_spheres(radius=outer_radius, region=region, pf=0.3, seed=124848351)

[1321] raise ValueError(f’Initial packing fraction {initial_pf} is greater than the limit for random sequential packing, {MAX_PF_RSP}')
#Calculate the sphere radius used in the initial random sequential
[1325] packing from the initial packing fraction
→ [1326] initial_radius = (3/4initial_pfdomain.volume/(pi*num_spheres))**(1/3)
[1327] domain.sphere_radius = initial_radius
[1329] #Recalculate the limits for the initial random sequential packing using
[1330] # the desired final sphere radius to ensure spheres are fully contained
[1331] # within the domain during the close random pack

ZeroDivisionError: float division by zero

I didn’t expect any error of this type. How can I solve it?
Thank you very much in advance.