problem with chart.TimeSeries()

I think there is a problem with the yearmon class in zoo when running
R 2.13 and higher. For a class project, I wanted students to make a
"growth of a $1" plot. Here is some sample code that didn't work in R
2.13.1 (works in R 2.12.2, however) 

I think there is a problem with the yearmon class in zoo when running R 2.13
and higher. For a class project, I wanted students to make a "growth of a
$1" plot. Here is some sample code that didn't work in R 2.13.1 (works in R
2.12.2, however)

options(digits=4, width=70)

# load packages

library("PerformanceAnalytics")

library("tseries")

start.date = "2006-05-01"

end.date = "2011-05-30"

vfinx.prices = get.hist.quote(instrument="vfinx", start=start.date,

+ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?end=end.date, quote="AdjClose",

+ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?provider="yahoo", origin="1970-01-01",

+ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?compression="m", retclass="zoo")

trying URL
'http://chart.yahoo.com/table.csv?s=vfinx&a=4&b=01&c=2006&d=4&e=30&f=2011&g=
m&q=q&y=0&z=vfinx&x=.csv'

Content type 'text/csv' length unknown

opened URL

downloaded 3007 bytes



time series ends ? 2011-05-02

# change time indices to class yearmon, which is most appropriate for

monthly data

index(vfinx.prices) = as.yearmon(index(vfinx.prices))

ReturnsCC.z = diff(log(vfinx.prices))

ReturnsSimple.z = exp(ReturnsCC.z) - 1

chart.CumReturns(ReturnsSimple.z, wealth.index=TRUE, legend.loc="topleft",

+ ? ? ? ? ? ? ? ? ?lwd=2, main="growth of $1")

Error in as.Date.default(time(y)) :

?do not know how to convert 'time(y)' to class "Date"

traceback()

5: stop(gettextf("do not know how to convert '%s' to class \"Date\"",

? ? ? deparse(substitute(x))))

4: as.Date.default(time(y))

3: as.Date(time(y))

2: chart.TimeSeries(Return.cumulative, col = colorset, legend.loc =
legend.loc,

? ? ? ...)

1: chart.CumReturns(ReturnsSimple.z, wealth.index = TRUE, legend.loc =
"topleft",

? ? ? lwd = 2, main = "growth of $1")