warning: singular in variogram fit

Dear R-sig-Geo,

I am trying to fit the the "Materon' model to my data with this command:

# define gstat object and compute variogram:
# mal = subset(mal, !is.na(mal$Zntot.s)) # needed if missing values
g = gstat(id = c("Zntot.s"), formula = Zntot.s~1, data = mal)
vg = variogram(g)
# vg = variogram(g, boundaries = c(15,25,45,70,100,150,200,250,300))
windows(width = 8, height = 6)
plot(vg, plot.numbers = TRUE)

# choose initial variogram model and plot:
vgm = vgm(nugget=0, psill=0.5, range=140, model="Mat", kappa = 0.9)
plot(vg, vgm)

# fit variogram model:
vgm = fit.variogram(vg, vgm)
windows(width = 8, height = 6)
plot(vg, vgm, main = "Variogram Zntoy.s")
vgm

Then i get this warning: singular model in variogram fit. What does it mean and how do i solve the problem? 

Thank you

 
Kind regards,

ISRAEL O. IKOYI
MSc. Thesis Research Student
Wageningen University 
Department of Soil Quality 
P.O. Box 47 
NL-6700 AA Wageningen 
The Netherlands

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