[R] sweave bwplot error

[Christoph Lehmann]

Hi
I use sweave and have a problem with the following figure, but not with 
other figures:

tt <- data.frame(c("a", "b", "c"), c(1.2, 3, 4.5))
names(tt) <- c("x1", "x2")
bwplot(x2 ~x1, data = tt)

ok now in sweave:

\begin{figure}[H]
   \begin{center}
<<echo=FALSE, fig=TRUE, height=5, width=10>>=
lset(col.whitebg())
bwplot(x2 ~x1, data = tt)
@
     \caption{xxx}
   \end{center}
\end{figure}

PROBLEM:
the pdf of the figure is not correctly created (neither the esp) and the 
error I get from sweave is:
pdf inclusion: required page does not exist <0>

thanks for help

christoph