[R] error in amer function

[Marilia malaria]

Dear all,

I am trying to reproduce the exemple in the vignette "Using lme4 to fit 
Generalized Additive
Mixed Models" with my dataset.
But...

 > mod <- amer(pasvig ~ -1 + harvf + tp(dias,by=harvf) + (1 | pac), 
data=exemplo)
Erro em if (from == to) rep.int(from, length.out) else as.vector(c(from,  :
  valor ausente onde TRUE/FALSE necessário

A subset of the dataset is attached.
No clue about what is wrong.

Thanks a lot!

Marilia