[R] problem with srt vector in xyplot {lattice}

[Wolfram Fischer - Z/I/M]

[ R 1.6.1 ]

PROBLEM
    The plot of the appended code does produce 
    a postscript file which is not interpretable
    by gv under Linux.

REMARK
    If the srt argument is commented out or set to a constant
	like 45 or 90, the ps file becomes interpretable.

CODE
    xytest <- function( ... ){
        with( airquality, { print( xyplot(
              Ozone ~ Temp | Month
            , panel = function( x, y, subscripts, ... ){
                ltext( x, y, Temp[subscripts]
                    , srt = 45 / max( Solar.R, na.rm=T ) * Solar.R
                    )
                }
            , ...
            )
        )})
    }

    xytest()        ## works!

    trellis.device( 'postscript', file = 'x.ps' )
    xytest()
    dev.off()


QUESTION
    Is there a workaround?


Wolfram