[R] Error message in polr

[german.lopez@ua.es]

Dear members of the list,
  I'm fitting ordinal regressions using polr, and in some models I 
get the error copied below. Dependent variable is an ordered factor 
of bird abundance categories, and predictors are continuous habitat 
variables.

> ro6 <- polr(formula = abun ~ InOmbrot + Oliva.OC + ToCultAr + 
DivCulArb + AltitMax + COORXY)
> summary(ro6)

Re-fitting to get Hessian

Error in La.svd(x, nu, nv, method) : error code 8 from Lapack 
routine dgesdd
In addition: Warning messages: 
1: NaNs produced in: dlogis(x, location, scale, log) 
2: NaNs produced in: dlogis(x, location, scale, log) 

But if I call for the "ro6" object I get the coefficients but 
without standard errors:

> ro6
Call:
polr(formula = abun ~ InOmbrot + Oliva.OC + ToCultAr + DivCulArb + 
    AltitMax + COORXY)

Coefficients:
     InOmbrot      Oliva.OC      ToCultAr     DivCulArb      
AltitMax 
-2.721242e-01  2.590153e-02  2.098157e-02  7.908437e-01  2.088895e-
03 
       COORXY 
 5.937940e-06 

Intercepts:
     0|1      1|2      2|3 
18.16425 18.97070 20.94103 

Residual Deviance: 817.667 
AIC: 835.667 

It seems that the error is related to the number of predictors 
involved since if I use a lower number, say five predictors, polr 
does not produce this error message in summary.
I have been looking at previous messages concerning errors in polr 
but I haven't found the solution. I would ackowledge very much any 
help.

  Germán López