Aggregate lag

hello,

Does anybody know how to aggregate a lag series ?
when I try to use aggregate I get the following message

try<-ts(1:100,start=c(1985,1),freq=12)
aggregate(try,4,mean,na.rm=T)

Qtr1 Qtr2 Qtr3 Qtr4
1985    2    5    8   11
1986   14   17   20   23
1987   26   29   32   35
1988   38   41   44   47
1989   50   53   56   59
1990   62   65   68   71
1991   74   77   80   83
1992   86   89   92   95
1993   98

aggregate(lag(try,-1),4,mean,na.rm=T)

Error in rep.int("", start.pad) : invalid number of copies in rep()

Matthieu