problem assigning POSIX class to AttributeList column

Hello, I've been unable to use "[[<-.AttributeList" to assign POSIXct vectors.

It seems to be because 
is.vector(Sys.time()) 

is FALSE.  The test "is.vector(value)" is perhaps too strict ?? but I
don't see whether this is perhaps a problem with the datetimeClasses. 

Example follows. 

Cheers, Mike.


sessionInfo()
#Version 2.3.1 (2006-06-01) 
#i386-pc-mingw32 

#attached base packages:
#[1] "methods"   "stats"     "graphics"  "grDevices" "utils"     "datasets" 
#[7] "base"     

#other attached packages:
#      sp 
#"0.8-16" 

al <- AttributeList(list(x = 1:10, y = gl(2, 5), t = Sys.time() + 1:10))


al[["t"]] <- Sys.time() + 1:10  
#Error: is.vector(value) || is.factor(value) is not TRUE

traceback()
#4: stop(paste(deparse(mc[[i + 1]]), " is not ", if (length(r) > 
#       1) "all ", "TRUE", sep = ""), call. = FALSE)
#3: stopifnot(is.vector(value) || is.factor(value))
#2: "[[<-.AttributeList"(`*tmp*`, "t", value = c(1153188132.868, 
#   1153188133.868, 1153188134.868, 1153188135.868, 1153188136.868, 
#   1153188137.868, 1153188138.868, 1153188139.868, 1153188140.868, 
#   1153188141.868))
#1: "[[<-"(`*tmp*`, "t", value = c(1153188132.868, 1153188133.868, 
#   1153188134.868, 1153188135.868, 1153188136.868, 1153188137.868, 
#   1153188138.868, 1153188139.868, 1153188140.868, 1153188141.868
#   ))


## the same problem occurs with
al[[3]] <- Sys.time() + 1:10

## and with

al[["t"]] <- al[["t"]]

## assigning numeric to numeric OK
al[["x"]] <- 2:11

## but assigning POSIX to same is not
al[["x"]] <- Sys.time() + 1:10


# straight list version

l <- list(x = 1:10, y = gl(2, 5), t = Sys.time() + 1:10)

## works fine:
l[["t"]] <- l[["t"]]