problem of implementing an effective 4-fermion model

Dear experts,

I am trying to implement a simple four fermions contact-like interaction.

I started with latest version of feynRules.

\feynRules\Models\SM.fr

adding the following new particle.

  F[16] == {
    ClassName -> chi,
    Indices -> {Index[Generation]},
    FlavorIndex -> Generation,
    SelfConjugate -> False,
    Mass -> {MNE1,200},
    Width -> 0,
    QuantumNumbers -> {Q -> 0} ,
    PropagatorLabel -> "chi",
    PropagatorType -> Straight,
    PropagatorArrow -> Forward,
    PDG -> 1000022,
    ParticleName -> "chi",
     FullName -> "chi",
     AntiParticleName -> "chi~"
  },

After I load the fr file, I define the model,

LDM := t tbar chi chibar
LDMSM := LSM + LDM

WriteUFO[LSMDM]

Then I copy the output directory/Standard_Model_UFO to MadGraph5_v1_5_9/models/

./bin/mg5

import model Standard_Model_UFO
INFO: load particles
INFO: load vertices
Command "import model Standard_Model_UFO" interrupted with error:
AttributeError : 'int' object has no attribute 'simplify'
Please report this bug on https://bugs.launchpad.net/madgraph5
More information is found in 'MG5_debug'.
Please attach this file to your report.

The new vertex is inside the file models/Standard_Model_UFO/vertices.py

V_7 = Vertex(name = 'V_7',
             particles = [ P.chi__tilde__, P.chi, P.t__tilde__, P.t ],
             color = [ '1' ],
             lorentz = [ L.FFFF1 ],
             couplings = {(0,0):C.GC_1})

the new lorentz is inside this file models/Standard_Model_UFO/lorentz.py

FFFF1 = Lorentz(name = 'FFFF1',
                spins = [ 2, 2, 2, 2 ],
                structure = '1')

The new particle is in the file: models/Standard_Model_UFO/particles.py

chi = Particle(pdg_code = 1000022,
               name = 'chi',
               antiname = 'chi~',
               spin = 2,
               color = 1,
               mass = Param.MNE1,
               width = Param.ZERO,
               texname = 'chi',
               antitexname = 'chi~',
               charge = 0,
               GhostNumber = 0,
               LeptonNumber = 0,
               Y = 0)

chi__tilde__ = chi.anti()

Do you have any ideas of what might be wrong? or any other suggestions?

I googled and found similar problem,
https://bugs.launchpad.net/madgraph5/+bug/947529

where Olivier has answered,

"Concerning the actual problem that you point (and that I can not reproduce), this error can occur only if
1) They are vertex a with at least one fermion.
2) The Lorentz structure is a number.
Such lorentz structure is not possible in presence of fermion, so this is probably due to a wrong model implementation (that's why I ask you to cross-check if you give me the correct model).
"

It seems that the problem is related to the lorentz structure. But how should I fix it?

Thanks,
Yong