multi sphere and volume_limit

Dear all,
I am confused with this error and need your help.
ERROR: Fix particletemplate/multisphere (id pts1): Volume expectancy too small. Change 'volume_limit' if you are sure you know what you're doing
what should I do for this error?
this is my code:
#Multisphere

atom_style sphere
atom_modify map array sort 0 0
boundary f f f
newton off

communicate single vel yes

#PUBLIC version only supports serial execution
processors 1 1 1

units si

region reg block -3.5 3.5 -2.5 2.5 -1.5 1.5 units box
create_box 1 reg

neighbor 0.004 bin
neigh_modify delay 0

#Material properties required for new pair styles

fix m1 all property/global youngsModulus peratomtype 1.e7
fix m2 all property/global poissonsRatio peratomtype 0.45
fix m3 all property/global coefficientRestitution peratomtypepair 1 0.3
fix m4 all property/global coefficientFriction peratomtypepair 1 0.5
fix m5 all property/global characteristicVelocity scalar 2.

#New pair style
pair_style gran model hertz tangential history #Hertzian without cohesion
pair_coeff * *

timestep 0.000005

fix gravi all gravity 9.81 vector 0.0 0.0 -1

fix zwalls all wall/gran model hertz tangential history primitive type 1 zplane -1.5
fix xwalls all wall/gran model hertz tangential history primitive type 1 xplane -3.5
fix xxwalls all wall/gran model hertz tangential history primitive type 1 xplane 3.5
fix ywalls all wall/gran model hertz tangential history primitive type 1 yplane 2.5
fix yywalls all wall/gran model hertz tangential history primitive type 1 yplane -2.5
#distributions for insertion
fix pts1 all particletemplate/multisphere 15485863 atom_type 1 density constant 2500 nspheres 17 ntry 1000000 spheres file data/stone1.multisphere scale 0.001 type 1
fix pdd1 all particledistribution/discrete 15485867 1 pts1 1.0

#region and insertion
region bc cylinder z -2.75 0 0.5 0.5 1.5 units box

fix ins all insert/pack seed 32452843 distributiontemplate pdd1 vel constant 0. 0. -1. &
insert_every once overlapcheck yes region bc ntry_mc 10000 volumefraction_region 0.4

#integrator for multisphere rigid bodies
fix integr all multisphere

#output settings, include total thermal energy
compute 1 all erotate/sphere
fix ts all check/timestep/gran 1000 0.1 0.1
thermo_style custom step atoms ke c_1 f_ts[1] f_ts[2] vol
thermo 1000
thermo_modify lost ignore norm no

run 1

#insert the first particles so that dump is not empty
dump dmp all custom/vtk 200 post/multi_*.vtk id type mol x y z vx vy vz fx fy fz omegax omegay omegaz radius

#insert particles
run 50000