[R] problem with the plm package (2)

[Giangiacomo Bravo]

Thanks a lot for the quick answerd I received and that helped me to 
solve my first (simple) problem with the plm package.

Unfortunatley, here's another one:
I do not know why, but I'm unable to estimate a simple model in the 
form y ~  1 + x

When I try
 > zz <- plm(y ~ 1 + x , data=mydata)
I obtain
Errore in X.m[, coef.within, drop = F] : numero di dimensioni errato
which means that the number of dimensions is wrong in X.m[, 
coef.within, drop = F].

However, I have no problem to estimate a more complex model, e.g.
 > zz <- plm(y ~ 1 + x + I(x^2), data=mydata)
in this case everything is ok.

What's wrong?

Thank you,
Giangiacomo