Conflict between xtable and Hmisc when using Sweave?

Dear R users,

The Sweave code below runs fine, as it is. However, an error occurs when 
the line 'library(xtable)' is uncommented:
Error:  chunk 1
Error in "label<-"(`*tmp*`, value = "month") :
        no applicable method for "label<-"

Is anybody aware of this and knows a workaround?

Thanks,

Sander.

*******************

\documentclass[a4paper]{article}
\title{Sweave Test for summarize}
\author{Sander Oom}

\usepackage{a4wide}

\begin{document}

\maketitle

\begin{figure}[ht]
\begin{center}
<<fig=TRUE,echo=FALSE>>=
  # library(xtable)
  library(Hmisc)
  set.seed(111)
  dfr <- expand.grid(month=1:12, year=c(1997,1998), reps=1:100)
  month <- dfr$month
  year <- dfr$year
  y <- abs(month-6.5) + 2*runif(length(month)) + year-1997
  s <- summarize(y, llist(month,year), smedian.hilow, conf.int=.5)
  print(xYplot(Cbind(y,Lower,Upper) ~ month, groups=year, data=s,
        keys='lines', method='alt', type='b'))
@
\end{center}
\end{figure}

\end{document}

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 > version

         _
platform i686-pc-linux-gnu
arch     i686
os       linux-gnu
system   i686, linux-gnu
status
major    2
minor    1.0
year     2005
month    04
day      18
language R