Extending time series

I noticed that when one tries to extend a time series (ts) object by a
single period using window() actually two observations (NAs) are
added. See below for an example. Does anyone know the reason for this
behavior and how to avoid it?

Thanks.

FS

x<-ts(c(1,2,3))
x1<- window(x, start(x), end(x) + 1, extend = TRUE)
x1

Time Series:
Start = 1 
End = 5 
Frequency = 1 
[1]  1  2  3 NA NA