Help with rounding legend intervals within a function

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library(cluster)
library(maptools)
library(RColorBrewer)
library(classInt)

#the function accepts a shapefile, a vector of values to plot, a
#number indicating the number of categories, and a title
myMap<-function(shape,plot.val,ncls=5,title="Generic Title"){
plotvar <- plot.val
nclr <- ncls
plotclr <- brewer.pal(nclr, "BuPu")
class <- classIntervals(plotvar, nclr, style = "quantile")
brks <- round(class$brks, 3)
brks[1]<-(floor(min(class$brks)*1000))/1000  #makes sure lower bound
        #is rounded down so it includes lowest value
brks[length(brks)]<-(ceiling(max(class$brks)*1000))/1000 #makes sure
        #upper bound is rounded up so it includes highest value
class <- classIntervals(plotvar, nclr, style = "fixed", fixedBreaks = brks)
colcode <- findColours(class, plotclr)

par(mar=c(0.5,0.5,0.5,0.5))
plot(shape, col = colcode)
legend("topleft", legend=names(attr(colcode, "table")),
 fill=attr(colcode, "palette"), cex = 0.75, bty = "n")
}

#Run the function
xx <- readShapePoly(system.file("shapes/sids.shp", package="maptools")[1],
IDvar="FIPSNO", proj4string=CRS("+proj=longlat +ellps=clrk66"))
plot(xx, border="blue", axes=TRUE, las=1)
names(xx)
dat <- cbind(xx$SID74, xx$NWBIR74, xx$BIR79)
fann <- fanny(dat, 3, maxit = 1000)

myMap(xx,fann$membership[,1],ncls=6)
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Running the function (on R 2.13) produces the error:
"Error in eval(expr, envir, enclos) : object 'brks' not found"

If I run traceback() I get:

5: eval(expr, envir, enclos)
4: eval(mc$...$fixedBreaks)
3: sort(eval(mc$...$fixedBreaks))
2: classIntervals(plotvar, nclr, style = "fixed", fixedBreaks = brks)
1: myMap(xx, fann$membership[, 1], ncls = 6)

what appears to be happening is that the eval expression is using the
main variable environment (if I run the code outside of the function
it works fine, or if I leave an object named brks in the main variable
environment it will see that variable and the function works).