FATAL ERROR: ISR/EPA handler: event does not contain two radiated particles

Hi,

I am trying to simulate the process e+e- -> t tbar, with each top decaying as t -> W b -> l nu b, i.e. dileptonic channel. I am getting the following error:

FATAL ERROR: ISR/EPA handler: event does not contain two radiated particles

I paste the .sin file below. I am using version 2.8.4. Thanks for your help!

# Select model
model = SM

# Define masses
mc = 0
ms = 0
mb = 4.7 GeV

me = 0
mmu = 0

# Define process (step number 1)
process ttbar = e1, E1 => t, tbar

process t_dec = t => Wp, b
process Wp_dec_1 = Wp => E1, n1
process Wp_dec_2 = Wp => E2, n2

process tbar_dec = tbar => Wm, B
process Wm_dec_1 = Wm => e1, N1
process Wm_dec_2 = Wm => e2, N2

compile

# Define energy (step number 2)
sqrts = 500 GeV

# Phase space parametrization (step number 3)
integrate (t_dec, Wp_dec_1, Wp_dec_2, tbar_dec, Wm_dec_1, Wm_dec_2)
integrate (ttbar) {iterations = 10:1000000:"gw", 5:500000:""}

# Define unstable particles and their corresponding decays (step number 4)
unstable t (t_dec)
unstable tbar (tbar_dec)
unstable Wp (Wp_dec_1, Wp_dec_2)
unstable Wm (Wm_dec_1, Wm_dec_2)

# Define beam spectrum and ISR
beams = e1, E1 => circe2 => isr

?isr_handler = true
$isr_handler_mode = "recoil"
isr_alpha = 0.0072993
isr_mass = 0.000510997 GeV

$circe2_file = "ilc500.circe"
$circe2_design = "ILC"
?circe2_polarized = false

# Define cuts
real default_M_cut = 4 GeV
real default_jet_cut = 10 GeV
real default_E_cut = 10 GeV
real default_Q_cut = 4 GeV
alias quarks = u:d:s:c:b:U:D:S:C:B
alias leptons = e1:e2:E1:E2

cuts = all M > default_M_cut [leptons, leptons]
       and all M > default_jet_cut [quarks, quarks]
       and all M < - default_Q_cut [incoming e1, e1]
       and all M < - default_Q_cut [incoming E1, E1]
       and all Pt > 5 GeV [quarks]

# Define parton shower and hadronization
?ps_fsr_active = true
?ps_isr_active = false
?hadronization_active = true
$shower_method = "PYTHIA6"
!?ps_PYTHIA_verbose = true

$ps_PYTHIA_PYGIVE = "MSTJ(28)=0; PMAS(25,1)=120.; PMAS(25,2)=0.3605E-02; MSTJ(41)=2; MSTU(22)=2000; PARJ(21)=0.40000; PARJ(41)=0.11000; PARJ(42)=0.52000; PARJ(81)=0.25000; PARJ(82)=1.90000; MSTJ(11)=3; PARJ(54)=-0.03100; PARJ(55)=-0.00200; PARJ(1)=0.08500; PARJ(3)=0.45000; PARJ(4)=0.02500; PARJ(2)=0.31000; PARJ(11)=0.60000; PARJ(12)=0.40000; PARJ(13)=0.72000; PARJ(14)=0.43000; PARJ(15)=0.08000; PARJ(16)=0.08000; PARJ(17)=0.17000; MSTP(3)=1;MSTP(71)=1"

# Define sample size
n_events = 1000

# Define output format
sample_format = lhef

# Simulate
simulate (ttbar)

And part of the relevant output:

...
........
n_events = 1000
| Starting simulation for process 'ttbar'
| Simulate: using integration grids from file 'ttbar.m1.vg'
| Simulate: activating ISR handler
| Simulate: ISR/EPA handler mode: pair recoil
| Simulate: activating decays
| Simulate: activating parton shower
     MSTU(12) changed from 0 to 12345
| Shower: Using PYTHIA6 shower
| Simulate: activating hadronization
| Hadronization: Using PYTHIA6 interface for hadronization and decays
| Simulate: using integration grids from file 'Wp_dec_1.m1.vg'
| Simulate: using integration grids from file 'Wp_dec_2.m1.vg'
| Simulate: using integration grids from file 't_dec.m1.vg'
| Simulate: using integration grids from file 'Wm_dec_1.m1.vg'
| Simulate: using integration grids from file 'Wm_dec_2.m1.vg'
| Simulate: using integration grids from file 'tbar_dec.m1.vg'
| RNG: Initializing TAO random-number generator
| RNG: Setting seed for random-number generator to 12520
| Simulation: requested number of events = 1000
| corr. to luminosity [fb-1] = 1.8235E+00
| Events: writing to LHEF file 'ttbar.lhe'
| Events: writing to raw file 'ttbar.evx'
| Events: generating 1000 unweighted, unpolarized events ...
| Events: event normalization mode '1'
 Particle set:
------------------------------------------------------------------------
 Particle 1 [i] f(11)
 E = 2.500000000000E+02
 P = 0.000000000000E+00 0.000000000000E+00 2.500000000000E+02
 T = 0.000000000000E+00
 Children: 3 4
 Particle 2 [i] f(-11)
 E = 2.500000000000E+02
 P = 0.000000000000E+00 0.000000000000E+00 -2.500000000000E+02
 T = 0.000000000000E+00
 Children: 3 4
 Particle 3 [o] f(6)c(1 )
 E = 2.500000000000E+02
 P = 8.917823768270E+00 -3.746694311532E+01 -1.762188712733E+02
 T = 2.996361000000E+04
 Parents: 1 2
 Particle 4 [o] f(-6)c(-1 )
 E = 2.500000000000E+02
 P = -8.917823768270E+00 3.746694311532E+01 1.762188712733E+02
 T = 2.996361000000E+04
 Parents: 1 2
******************************************************************************
******************************************************************************
*** FATAL ERROR: ISR/EPA handler: event does not contain two radiated particles
******************************************************************************
******************************************************************************
| There were no errors and 7 warning(s).
WHIZARD run aborted.