[SOLVED] BehaviorSpace freezes on step #0

Sorry if this is obvious, I have searched everything I can think of and asked a colleague and we are both stuck.

I have some code (below - a slight twist on wolf sheep predation) that I want to run behaviorspace (input also below). However, it never runs to completion. It always seems to get stuck on step #0 of a run (the run # it gets stuck on varies). The runs that it sticks on work fine if they are run in isolation (so all the variables seem fine). It just seems to freeze (producing no error messages, nor does it crash).

Any ideas what is causing it?

Thanks,

Simon

Code:

globals [grass]  ;; keep track of how much grass there is
;; Sheep and wolves are both breeds of turtle.
breed [sheep a-sheep]  ;; sheep is its own plural, so we use "a-sheep" as the singular.
breed [wolves wolf]
turtles-own [energy]       ;; both wolves and sheep have energy
patches-own [countdown]

to setup
  clear-all
  ask patches [ set pcolor green ]
  ;; check GRASS? switch.
  ;; if it is true, then grass grows and the sheep eat it
  ;; if it false, then the sheep don't need to eat
  if grass? [
    ask patches [
      set countdown random grass-regrowth-time ;; initialize grass grow clocks randomly
      set pcolor one-of [green brown]
    ]
  ]
  if fences?[ ; this code adds in blue fences to create patches of various size (if fences are turned on). Turtles cannot pass over fences
      ask patches with [pxcor mod connectivity2 = 0]
          [ set pcolor blue ]
      ask patches with [pycor mod connectivity = 0]
          [ set pcolor blue ]
   ]
  set-default-shape sheep "sheep"
  create-sheep ((count patches - (count patches with [pcolor = blue])) / 25)  ;; create the sheep, then initialize their variables. The starting density always remains constant despite a varying world size
  [
    set color white
    set size 1.5  ;; easier to see
    set label-color blue - 2
    set energy random (2 * sheep-gain-from-food)
    setxy random-xcor random-ycor
  ]
  set-default-shape wolves "wolf"
  create-wolves ((count patches - (count patches with [pcolor = blue])) / 50)   ;; create the wolves, then initialize their variables. The starting density always remains constant despite a varying world size
  [
    set color black
    set size 2  ;; easier to see
    set energy random (2 * wolf-gain-from-food)
    setxy random-xcor random-ycor
  ]
  display-labels
  set grass count patches with [pcolor = green]
  reset-ticks
end

to go
  if count wolves = 0 and count sheep = 0 and ((count patches with [pcolor = green]) = (count patches - (count patches with [pcolor = blue]))) [ stop ] ; stop model when the whole world has flipped to grass
  ask sheep [
    move
    if grass? [
      set energy energy - 1  ;; deduct energy for sheep only if grass? switch is on
      eat-grass
    ]
    death
    reproduce-sheep
  ]
  ask wolves [
    move
    set energy energy - 1  ;; wolves lose energy as they move
    catch-sheep
    death
    reproduce-wolves
  ]
  if grass? [ ask patches [ grow-grass ] ]
  set grass count patches with [pcolor = green]
  tick
  display-labels
end

to move  ;; turtle procedure
  rt random 50
  lt random 50
    ifelse [pcolor] of patch-ahead 1 = blue
      [ move]   ;; If there is a blue patch ahead of you, choose another random direction
      [ fd 1 ]                  ;; Otherwise, it is safe to move forward.
end

to eat-grass  ;; sheep procedure
  ;; sheep eat grass, turn the patch brown
  if pcolor = green [
    set pcolor brown
    set energy energy + sheep-gain-from-food  ;; sheep gain energy by eating
  ]
end

to reproduce-sheep  ;; sheep procedure
  if random-float 100 < sheep-reproduce [  ;; throw "dice" to see if you will reproduce
    set energy (energy / 2)                ;; divide energy between parent and offspring
    hatch 1 [ ifelse [pcolor] of patch-ahead 1 = blue ;; hatch an offspring
      [ move]   ;; If there is a blue patch ahead of the offspring, it chooses another random direction
      [ fd 1 ]  ;;If not, offspring move forward 1 step
    ]]
end

to reproduce-wolves  ;; wolf procedure
  if random-float 100 < wolf-reproduce [  ;; throw "dice" to see if you will reproduce
    set energy (energy / 2)               ;; divide energy between parent and offspring
    hatch 1 [ ifelse [pcolor] of patch-ahead 1 = blue ;; hatch an offspring
      [ move  ] ;; If there is a blue patch ahead of the offspring, it chooses another random direction
      [ fd 1 ] ;;If not, offspring move forward 1 step
      ] ]
end

to catch-sheep  ;; wolf procedure
  let prey one-of sheep-here                    ;; grab a random sheep
  if prey != nobody                             ;; did we get one?  if so,
    [ ask prey [ die ]                          ;; kill it
      set energy energy + wolf-gain-from-food ] ;; get energy from eating
end

to death  ;; turtle procedure
  ;; when energy dips below zero, die
  if energy < 0 [ die ]
end

to grow-grass  ;; patch procedure
  ;; countdown on brown patches: if reach 0, grow some grass
  if pcolor = brown [
    ifelse countdown <= 0
      [ set pcolor green
        set countdown grass-regrowth-time ]
      [ set countdown countdown - 1 ]
  ]
end

to display-labels
  ask turtles [ set label "" ]
  if show-energy? [
    ask wolves [ set label round energy ]
    if grass? [ ask sheep [ set label round energy ] ]
  ]
end

; Copyright 1997 Uri Wilensky.
; See Info tab for full copyright and license.

behaviorspace input:

variables:

["world-width" 51]
["fences?" true]
["wolf-gain-from-food" 20]
["sheep-reproduce" 7]
["show-energy?" false]
["grass?" true]
["connectivity" 10 25 50]
["wolf-reproduce" 5]
["grass-regrowth-time" [1 1 100]]
["sheep-gain-from-food" 4]

repetitions: 1

reporters:

count wolves
count sheep
count patches with [pcolor = green]

measure runs at each step

setup: setup

Go: go

Stop (I have tried it with and without this): count wolves = 0 and count sheep = 0 and ((count patches with [pcolor = green]) = (count patches - (count patches with [pcolor = blue])))

Time limit: 5000