[R] xts/zoo index problem?

[ce]

Dear all,

I have this code :

library(xts)
a <- structure(c(1,2), class = c("xts", "zoo"), .indexCLASS = c("POSIXct", 
"POSIXt"), .indexTZ = "", tclass = c("POSIXct", "POSIXt"), tzone = "", index = structure(c(1453137885.23, 
1453149114.079), tzone = "", tclass = c("POSIXct", "POSIXt")), .Dim = c(2L, 
1L), .Dimnames = list(NULL, "value"))

a

a["2016-01-18 12:24:45.230"]
#                    value
#2016-01-18 12:24:45     1

a["2016-01-18 15:31:54.078"]
#                   value

Why second line doesn't show the value? Something to do with miliseconds ?
Thanks 
CE