CHOLMOD warning: matrix not positive definite

Hi,
I'd like to use PeriodicFlowEngine,and I test it in below script.When the PeriodicFlowEngine is actived,I got the warning:

CHOLMOD warning: matrix not positive definite. file: ../Supernodal/t_cholmod_super_numeric.c line: 911
something went wrong in Cholesky factorization, use LDLt as fallback this time1

Is there any error in my script,or something wrong with Yade？
Thanks for your any suggestions.

##############################
from __future__ import print_function
from yade import pack,qt
O.periodic=True

O.cell.hSize=Matrix3(0.1, 0, 0,
    0 ,0.1, 0,
    0, 0, 0.1)

sp=pack.SpherePack()
radius=5e-3
num=sp.makeCloud(Vector3().Zero,O.cell.refSize,radius,.2,500,periodic=True) # min,max,radius,rRelFuzz,spheresInCell,periodic
O.bodies.append([sphere(s[0],s[1]) for s in sp])

O.engines=[
 ForceResetter(),
 InsertionSortCollider([Bo1_Sphere_Aabb()],verletDist=.05*radius),
 InteractionLoop(
  [Ig2_Sphere_Sphere_ScGeom()],
  [Ip2_FrictMat_FrictMat_FrictPhys()],
  [Law2_ScGeom_FrictPhys_CundallStrack()]
 ),
 PeriTriaxController(dynCell=True,mass=0.2,maxUnbalanced=0.01,relStressTol=0.02,goal=(-1e4,-1e4,0),stressMask=3,globUpdate=5,maxStrainRate=(1.,1.,1.),doneHook='triaxDone()',label='triax'),
 NewtonIntegrator(damping=.2),
    PeriodicFlowEngine(dead=1,label='flow')
]
O.dt=PWaveTimeStep()
O.run();
qt.View()

phase=0

flow.meshUpdateInterval=200
flow.useSolver=3
flow.permeabilityFactor=1
flow.bndCondIsPressure=[1,1,0,0,0,0]
flow.bndCondValue=[0,1,0,0,0,0]

def triaxDone():
    triax.dead=1
    flow.dead=0
    flow.updateTriangulation=True
    O.pause()
##############################