R code not working under R version 2.13 for Mac

Hello,

I had a piece of code regularly working on R 2.12 now giving error on the newly installed R 2.13:

d5 <- aggregate(d4[,c("ore")],list(d4$progetto),sum)

Errore in units == units[1L] : confronto tra questi tipi non implementato - that's basically saying "comparison between these types has not been implemented"

How can I solve this issue? Please find attached some info about data frame d4

class(d4)

[1] "data.frame"

names(d4)

[1] "data"      "anno"      "mese"      "settimana" "progetto"  "ore"

class(d4$ore)

[1] "difftime"

class(d4$progetto)

[1] "factor"

Also, this is the R version I have installed:

R version 2.13.0 (2011-04-13)
Copyright (C) 2011 The R Foundation for Statistical Computing
ISBN 3-900051-07-0
Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)

Thanks,
Luca