[R-sig-Geo] GLS spatial model-- range error

[jofre carnicer cols]

Hello,

I am computing a gls spatial model:

require(nlme)

gls.exp<-gls(S1 ~ X2TPall + tmngem + tmaxjlm + arbdenkm + prat_herm + 
    arbclakm + conreukm, data=snouter.df, correlation=corExp(form~X+Y), na.action=na.omit)

and obtain the following error:

Error en Initialize.corSpatial(X[[1L]], ...) : 
  Range must be > 0 in "corSpatial" initial value

I have checked CorExp, gls, and CorClasses help but couldn't find how to solve this error. Any help is welcome.

Many thanks in advance,

Jofre