R help - Dec 2017 - t.trellis doesn't invert

> version
               _
platform       x86_64-w64-mingw32
arch           x86_64
os             mingw32
system         x86_64, mingw32
status         Patched
major          3
minor          4.3
year           2017
month          12
day            12
svn rev        73903
language       R
version.string R version 3.4.3 Patched (2017-12-12 r73903)
nickname       Kite-Eating Tree
> tt <- xyplot(y ~ x | a*b, data=data.frame(x=1:6, y=7:12, a=c(1:3,1:3),
b=c(1,1,1,2,2,2)))
> tt
> t(tt)
> t(t(tt))
Error in update.trellis(x, perm.cond = rev(x$perm.cond)) :
  Invalid value of 'perm.cond'
>

compare to numeric matrix
> tmp <- cbind(1:3, 4:6)
> tmp
     [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6
> t(tmp)
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6
> t(t(tmp))
     [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6
>