how to use clumps

When I run the following script

from yade import pack
spheremat=O.materials.append(FrictMat(density=2600,young=150e6,poisson=0.5,frictionAngle=30))
a=[]
for i in range(3):
 a.append(pack.SpherePack())
 r=0.03*i+0.02
 a[i].add((0,0,0),r)
 a[i].add((0,0,1.5*r),r)

sp=pack.SpherePack()
sp.makeClumpCloud((0,0,0),(2,2,10),a)
sp.toSimulation(material=spheremat)
wallmat=O.materials.append(FrictMat(density=1000,young=150e6,poisson=0,frictionAngle=0))
O.bodies.append(utils.facetBox((1,1,5),(1,1,5),wallMask=31,material=wallmat))

O.engines=[
 ForceResetter(),
 InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Facet_Aabb()]),
 InteractionLoop(
  [Ig2_Sphere_Sphere_ScGeom(),Ig2_Facet_Sphere_ScGeom()],
  [Ip2_FrictMat_FrictMat_FrictPhys()],
  [Law2_ScGeom_FrictPhys_CundallStrack()]
 ),
 GravityEngine(gravity=(0,0,-9.81)),
 NewtonIntegrator(damping=0.4),
]
O.dt=.5*utils.PWaveTimeStep()

I get
Body #2570 has velocity==NaN!
What's wrong

Thanks