trouble with FieldSaver commands

I'm having trouble with the FieldSaver commands. I'm currently running rot_compress.py; code below

 # rot_compress.py: A uniaxial compression simulation using ESyS-Particle
# Author: D. Weatherley
# Date: 27 December 2008
# Organisation: ESSCC, University of Queensland
# (C) All rights reserved, 2008.
#
#
#import the appropriate ESyS-Particle modules:
from esys.lsm import *
from esys.lsm.util import *
from esys.lsm.geometry import *
from WallLoader import WallLoaderRunnable
#from POVsnaps import POVsnaps

#instantiate a simulation object and initialise the neighbour search algorithm
sim = LsmMpi(numWorkerProcesses=1, mpiDimList=[1,1,1])

# initialise the neighbour search algorithm
sim.initNeighbourSearch (particleType = "RotSphere", gridSpacing = 5.0000, verletDist = 0.08000)

# set the number of timesteps and timestep increment:
sim.setNumTimeSteps(250000)
sim.setTimeStepSize(1.0000e-06)

# specify the spatial domain for the simulation:
domain = BoundingBox(Vec3(-20,-20,-20), Vec3(20,20,20))
sim.setSpatialDomain(domain)

# create a prism of spherical particles
geoRandomBlock = RandomBoxPacker (minRadius = 0.400, maxRadius = 2.0000, cubicPackRadius = 2.2000, maxInsertFails = 5000, bBox = BoundingBox(Vec3(-5.0000, 0.0000, -5.0000), Vec3(5.0000, 20.0000, 5.0000)), circDimList = [False, False, False], tolerance = 1.0000e-05)
geoRandomBlock.generate()
geoRandomBlock_particles = geoRandomBlock.getSimpleSphereCollection()

# add the particles to the simulation object
sim.createParticles(geoRandomBlock_particles)

# bond particles together with bondTag = 1
sim.createConnections(ConnectionFinder(maxDist = 0.005, bondTag = 1, pList = geoRandomBlock_particles))

# create a wall at the bottom of the model
sim.createWall(name = "bottom_wall", posn = Vec3(0.0000, 0.0000, 0.0000), normal = Vec3(0.0000, 1.0000, 0.0000))

# create a wall at the top of the model
sim.createWall(name = "top_wall", posn = Vec3(0.0000, 20.0000, 0.0000), normal = Vec3(0.0000, -1.0000, 0.0000))

# create rotational elastic-brittle bonds between particles
pp_bonds = sim.createInteractionGroup (BrittleBeamPrms(name = "pp_bonds", youngsModulus = 100000.0, poissonsRatio = 0.25, cohesion = 100.0, tanAngle = 1.0, tag = 1))

# initialise frictional interactions for unbonded particles
sim.createInteractionGroup (FrictionPrms(name="friction", youngsModulus = 100000.0, poissonsRatio = 0.25, dynamicMu = 0.4, staticMu = 0.6))

# create an exclusion between bonded and frictional interactions
sim.createExclusion (interactionName1 = "pp_bonds", interactionName2 = "friction")

# specify elastic repulsion from the bottom wall
sim.createInteractionGroup(NRotElasticWallPrms(name="bottom_wall_repel", wallName = "bottom_wall", normalK=1.000000e+05))

# specify elastic repulsion from the top wall
sim.createInteractionGroup(NRotElasticWallPrms(name="top_wall_repel", wallName = "top_wall", normalK=1.000000e+05))

# add translational viscous damping
sim.createInteractionGroup(LinDampingPrms(name="damping1", viscosity = 0.002, maxIterations = 50))

# add rotational viscous damping
sim.createInteractionGroup(RotDampingPrms(name="damping2", viscosity = 0.002, maxIterations = 50))

# add a wall loader to move the top wall
wall_loader1 = WallLoaderRunnable (LsmMpi = sim, wallName = "top_wall", vPlate = Vec3(0.0, -0.125, 0.0), startTime = 0, rampTime = 50000)
sim.addPreTimeStepRunnable (wall_loader1)

# add a wall loader to move the bottom wall
wall_loader2 = WallLoaderRunnable (LsmMpi = sim, wallName = "bottom_wall", vPlate = Vec3(0.0, 0.125, 0.0), startTime = 0, rampTime = 50000)
sim.addPreTimeStepRunnable (wall_loader2)

# create a FieldSaver to store number of bonds
#sim.createFieldSaver (InteractionScalarFieldSaverPrms(interactionName = "pp_bonds", fieldName = "count", fileName = "nbonds.dat", fileformat = "SUM", beginTimeStep = 0, endTimeStep = 250000, timeStepIncr = 1))

# create a FieldSaver to store the total kinetic energy of the particles
#sim.createFieldSaver (ParticleScalarFieldSaverPrms(fieldName = "e_kin", fileName = "ekin.dat", fileformat = "SUM", beginTimeStep = 0, endTimeStep = 250000, timeStepIncr = 1))

# create a FieldSaver to store potential energy stored in bonds
#sim.createFieldSaver (ParticleScalarFieldSaverPrms(interactionName = "pp_bonds", fieldName = "potential_energy", fileName = "epot.dat", fileformat = "SUM", beginTimeStep = 0, endTimeStep = 250000, timeStepIncr = 1))

# create a FieldSaver for wall positions
posn_saver = WallVectorFieldSaverPrms(wallName = ["bottom_wall", "top_wall"], fieldName = "Position", fileName = "out_Position.dat", fileformat = "RAW_SERIES", beginTimeStep = 0, endTimeStep = 250000, timeStepIncr = 10)
sim.createFieldSaver(posn_saver)

# create a FieldSaver for wall forces
#force_saver = WallVectorFieldSaverPrms (wallName = ["bottom_wall", "top_wall"], fieldName = "Force", fileName = "out_Force.dat", fileformat = "RAW_SERIES", beginTimeStep = 0, endTimeStep = 250000, timeStepIncr = 10)
#sim.createFieldSaver(force_saver)

#execute the simulation
sim.run()

(I've commented out all the FieldSaver commands except the one for wall position -- however, none of them work).

I get the error message below:

rsarracino@ccore-rsarracino:~/dem_routines/rock$ mpirun -np 2 /usr/local/bin/esysparticle rot_compress.py
CSubLatticeControler::initMPI()
slave started at local/global rank 0 / 1
Traceback (most recent call last):
  File "rot_compress.py", line 85, in <module>
    posn_saver = WallVectorFieldSaverPrms(wallName = ["bottom_wall", "top_wall"], fieldName = "Position", fileName = "out_Position.dat", fileformat = "RAW_SERIES", beginTimeStep = 0, endTimeStep = 250000, timeStepIncr = 10)
Boost.Python.ArgumentError: Python argument types in
    WallVectorFieldSaverPrms.__init__(WallVectorFieldSaverPrms)
did not match C++ signature:
    __init__(_object*, boost::python::list wallName, std::string fieldName, std::string fileName, std::string fileFormat, int beginTimeStep, int endTimeStep, int timeStepIncr)
    __init__(_object*, boost::python::tuple wallName, std::string fieldName, std::string fileName, std::string fileFormat, int beginTimeStep, int endTimeStep, int timeStepIncr)
    __init__(_object*, std::string wallName, std::string fieldName, std::string fileName, std::string fileFormat, int beginTimeStep, int endTimeStep, int timeStepIncr)

If I comment out all the FieldSaver commands, the program runs w/o problems.