Error during exponentiation: “math operation produced a non-number” - runtime-error

Question

during the execution of this code the model run in the error reported in the object. In a previous model where the variable land-sustainability was setting from 1 to 6 I
didn't incurred in this kind of error. I listed the code below reporting the portion of the code highlithed by the error at the end:
turtles-own [
profit-from-fuel
profit-from-food
expected-fuel-sell-price
expected-food-sell-price
profit
farm
farm-size
]
patches-own [
fuel-yeld
food-yeld
land-sustainability
water-level
belongs-to
food
fuel
]
globals [
fuel-sell-price
food-sell-price
governs
]
to setup
clear-all
clear-all-plots
create-farmers
setup-land
reset-ticks
ask turtles
[ set-farm-in-radius farm-size ]
set fuel-sell-price 184 + random 55 + random -55
set food-sell-price 184 + random 55 + random -55
end
to create-farmers
create-turtles 30
[
set shape "person farmer"
setxy random-pxcor random-pycor
set profit-from-fuel 0 ; indicizzazione del profitto iniziale a 0
set profit-from-food 0 ; indicizzazione del profitto iniziale a 0
set farm-size random 5 + 1
set label farm-size
]
end
to setup-land
ask patches [set belongs-to nobody]
ask patches
[
set pcolor 3
set food-yeld 13000
set fuel-yeld 13000
set land-sustainability random-float 0.9 + 0.1 ; per creare un moltiplicatore che vada da 0.1 a 1
set water-level random 3 * 1000
if water-level = 0 [set water-level 1000]
]
end
to set-farm-in-radius [d]
move-to one-of patches with [not any? other patches in-radius d with [belongs-to != nobody]]
set farm patches in-radius farm-size
ask farm [set belongs-to myself]
let c random 6 + 61
ask farm [set pcolor c]
end
to set-farm-distance [d]
move-to one-of patches with [not any? other patches with [belongs-to != nobody and distance myself < d]]
set farm patches with [distance myself < d]
ask farm [set belongs-to myself]
let c random 6 + 61
ask farm [set pcolor c]
end
to go
ask turtles [
set expected-fuel-sell-price fuel-sell-price + random 5 + random -5
if expected-fuel-sell-price < 0 [set expected-fuel-sell-price 1]
set expected-food-sell-price food-sell-price + random 5 + random -5
if expected-food-sell-price < 0 [set expected-food-sell-price 1]
set profit profit-from-fuel + profit-from-food
if profit = 0 [ set profit 1 ]
]
ask turtles [
cultivate
ask farm [recolor-farm]
set profit profit-from-food + profit-from-fuel
]
set fuel-sell-price fuel-sell-price + random 55 + random -55
if fuel-sell-price < 126 [set fuel-sell-price 126 ]
if fuel-sell-price > 245 [set fuel-sell-price 245] ; è corretta come assunzione? su un arco di 30 anni è ragionevole suppore che il limite massimo del prezzo
set food-sell-price food-sell-price + random 55 + random -55 ; sia di soli 5 euro più alto rispetto al massimo delgi ultimi 10 anni
if food-sell-price < 126 [set food-sell-price 126]
if food-sell-price > 245 [set food-sell-price 245]
if ticks = Duration [ stop ]
if ticks > Duration [stop]
ask patches
[ set food 0
set fuel 0
if land-sustainability < 0.1 [set land-sustainability 0.1] ; all'interno del modello la land susitability varia in base alle colture utilizzate
if land-sustainability > 1 [set land-sustainability 1 ] ; ma questa non può comunque essere inferiore a 0.1 o maggiore di 1
]
tick
end
to cultivate
let r risk-attitude
let e-f expected-fuel-sell-price
let e-food expected-food-sell-price
ask farm [
ifelse land-sustainability < 0.35 or water-level < 1100 ; 0.35 è un valore arbitrario che può essere modificato vedere i documenti FAO
[ ask myself [set profit-from-food sum [food-sell-price * (((food-yeld ) ^ (alfa)) * (((water-level) ^ (1 - gamma)) * ((land-sustainability) ^ (gamma))))] of farm
]
set food 2
set fuel 0
set land-sustainability land-sustainability + 0.1 ; la scelta di coltivare prodotti destinati alla produzione di cibo incrementa il livello l-s del terreno così coltivato
set pcolor 52.5
]
[
if land-sustainability >= 0.35 or water-level = 0
[
let utility-from-food e-food * ((food-yeld ) ^ (alfa)) * ((water-level) ^ (gamma)) * ((land-sustainability) ^ (beta)) ; da riscrivere la nuova formula con l'utilità di
let utility-from-fuel e-f *(( fuel-yeld) ^ (alfa) * ((water-level) ^ (1 - gamma) * (land-sustainability) ^ (gamma) ))
ifelse utility-from-food < utility-from-fuel
[
ask myself [set profit-from-fuel sum [fuel-sell-price * (((fuel-yeld ) ^ (alfa)) * (((water-level) ^ (1 - gamma)) * ((land-sustainability) ^ (gamma))))] of farm ] ; ricominciare da qui cobb-douglas
set fuel 1
set food 0
set land-sustainability land-sustainability - 1
]
[ ask myself [set profit-from-food sum [food-sell-price * (((food-yeld ) ^ (alfa)) * (((water-level) ^ (1 - gamma)) * ((land-sustainability) ^ (gamma))))] of farm ]
set food 1
set fuel 0
set land-sustainability land-sustainability + 0.5
]
]
]
]
end
to recolor-farm
if (food = 1 and fuel = 0 ) [set pcolor green ]
if (fuel = 1 and food = 0 )[set pcolor yellow]
if (fuel = 1 and food = 1) [set pcolor pink] ; check
if (fuel = 0 and food = 0) [set pcolor blue] ; check
end
[ ask myself [set profit-from-food sum [food-sell-price * (((food-yeld ) ^ (alfa)) * (((water-level) ^ (1 - gamma)) * ((land-sustainability) ^ (gamma))))] of farm

Answer 1

a ^ b will produce this error message if a is negative and b is fractional, so for example if you try to take the square root of -1:
observer> show -1 ^ 0.5
ERROR: Runtime error: math operation produced a non-number
Trying adding show land-sustainability and show gamma before the set profit-from-food ... line, to see what the bad values are, exactly, that you're trying to exponentiate. Then you'll have some debugging to do to figure out where those bad values are coming from.
Probably you're letting land-sustainability become negative.