groupe averages error

I want to get subsets averaged on numeric variables number 5:13 but I 
get the error below that I cannot understand.

is.factor(dat$Statut)

[1] TRUE

s<-ave(dat[,5:13],dat$Statut)

Error in [<-.data.frame(*tmp*, , value = if (n) { : 
        Argument "i" is missing, with no default

Thanks in advance
---------------
Charles RAUX,
Laboratoire d'Economie des Transports
CNRS-Universit? Lumi?re Lyon 2-ENTPE
email : charles.raux at let.ish-lyon.cnrs.fr
http://www.ish-lyon.cnrs.fr/let
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