Error in function (classes, fdef, mtable)

Hi all

I?m trying to overlay a set of polygons over another set of polygons and
calculate area of overlap. However, I?m getting the error:

"Error in function (classes, fdef, mtable)  :  unable to find an inherited
method for function ?is.projected? for signature ?"NULL???

What am I doing wrong here? I realise this is likely to be a question
regarding the fundamentals of coordinate reference systems, but after hours
of googling, I cannot find a fix. My code looks like this:


library(raster); library(sp); library(maptools); library(plyr);
library(rgeos); library(rgdal)

# make raster and convert to polygons
sectors <- raster(ext=extent(8, 9, 12, 13), ncol=5, nrow=5, crs=NA)
sectorsPoly <- rasterToPolygons(sectors)
sectorsPoly at data$value <- 1:25

# import shapefiles
setwd(system.file("shapes", package="maptools"))
columbus <- readShapeSpatial("columbus.shp")

# assign coordinate system
proj4string(sectorsPoly) <- CRS("+proj=longlat +datum=WGS84 +ellps=WGS84
+towgs84=0,0,0")
proj4string(columbus) <- CRS("+proj=longlat +datum=WGS84 +ellps=WGS84
+towgs84=0,0,0")

# project
sectorsPoly <- spTransform(sectorsPoly, CRS("+init=epsg:26392
+datum=WGS84"))
columbus <- spTransform(columbus, CRS("+init=epsg:26392 +datum=WGS84"))

# calculate proportion of sectorsPoly that is overlapped by columbus
llply(1:25, function(x) gArea(gIntersection(sectorsPoly, columbus[x,],
byid=TRUE), byid=TRUE)/gArea(sectorsPoly[x,]))

Thanks
Ross