error in using by + median

dear all,
Could anyone explain me the behaviour of median() within by()?
(I am running R.2.7.0)

thanks,
vito

 > H<-cbind(rep(0:1,l=20),matrix(rnorm(20*2),20,2))
 > by(H[,-1],H[,1],mean)
INDICES: 0
         V1         V2
-0.2101069  0.2954377
--------------------------------------------------------------------------------------------------------------------- 

INDICES: 1
          V1          V2
-0.23682173 -0.01225147
 > by(H[,-1],H[,1],median)
Error in median.default(data[x, , drop = FALSE], ...) : need numeric data
 >

====================================
Vito M.R. Muggeo
Dip.to Sc Statist e Matem `Vianelli'
Universit? di Palermo
viale delle Scienze, edificio 13
90128 Palermo - ITALY
tel: 091 6626240
fax: 091 485726/485612
http://dssm.unipa.it/vmuggeo