[R] nlme, predict.nlme, levels not allowed

[Patrick Giraudoux]

Dear listers,

I am trying to fit a nlme model with "age" and "pds" as reals, and 
"zone" a factor  with two levels "Annaba" and "Boumalek" . The "best" 
model found is the following:

 > modm3
Nonlinear mixed-effects model fit by maximum likelihood
  Model: pds ~ Asym/(1 + exp((xmid - age)/scal))
  Data: croispulm
  Log-likelihood: -91.86667
  Fixed: list(Asym ~ zone, xmid ~ zone, scal ~ 1)
 Asym.(Intercept) Asym.zoneBoumalek  xmid.(Intercept) 
xmid.zoneBoumalek              scal
       9.99551079        0.39423966        4.97981027        
0.06969807        2.23116661

Random effects:
 Formula: list(Asym ~ 1, xmid ~ 1)
 Level: nichoir
 Structure: General positive-definite, Log-Cholesky parametrization
                 StdDev       Corr 
Asym.(Intercept) 1.796565e-06 As.(I)
xmid.(Intercept) 1.219400e-04 0    
Residual         6.163282e-01      

Correlation Structure: Continuous AR(1)
 Formula: ~age | nichoir
 Parameter estimate(s):
      Phi
0.3395242
Number of Observations: 102
Number of Groups: 17


Everything normal so far.

Things come to be strange when I try to compute predicted values:

 > pred<-predict(modm3,newdata=mydata,type="response")
Error in predict.nlme(modm3, newdata = mydata, type = "response") :
        Levels Annaba,Boumalek not allowed for zone

I have checked and re-checked that zone in the newdata is well a factor 
with the "good" levels, and I can hardly understand why these two levels 
used when fitting the model are now rejected when used for computing 
predicted values.

Any hint welcome,

Best regards,

Patrick