[R] Cause of Error 1:nrow(X) : argument NA / NaN

[Jean-Louis Abitbol]

Dear R Helpers

I am trying to get function smedian.hilow to work using Hmisc summarize
on variable conc in dataframe pkindivmtd by time and dose using:

attach(pkindivmtd)
sconc <- summarize(conc,llist(time,dose),smedian.hilow)

I get the error message

Erreur dans 1:nrow(X) : argument NA / NaN

Contents of pkindivmtd is 

> contents(pkindivmtd)

Data frame:pkindivmtd   1296 observations and 9 variables    Maximum #
NAs:0

                Labels Units Levels Storage
dose              Dose    mg        integer
day                                 integer
hour                                integer
subject        Subject              integer
conc     Concentration ng/mL         double
fday                             10 integer
fsubject                         37 integer
fdose             Dose            4 integer
time              Time hours        integer

.....
> 

What I am doing wrong ?

Sorry for not providing a reproducible example as when I generate
randomly the 3 variables it works ! 

Many thanks,

Jean-Louis