[R] how to add a box to map plotted by a levelplot?

[Jonsson]

conne2 <- file("C:\\ome1440s.bin","rb")
         bioms<- readBin(conne2, integer(), size=1,  n=1440*720, signed=F)
        library(raster)
        library(rasterVis)
        data(wrld_simpl)
      library(maptools) ## needed for wrld_simpl
        r <- raster(nrow=720, ncol=1440)
        r[] <- bioms;    r <- ratify(r);    rat <- levels(r)[[1]];   
rat$soil <- LETTERS[1:13];     levels(r) <- rat
         myPal <- c('gray80','gray80','blanchedalmond'
,'chartreuse','yellow','navajowhite2','salmon','lightskyblue','brown4','orange','burlywood4','palegreen','forestgreen')
           
         levelplot(r, col.regions=myPal) +
            layer(sp.polygons(wrld_simpl, lwd=0.5))
 
I want to add a box representing this zone of latitude and longtitude:

             e6 <- extent( 2  , 8 , 45   , 51  )#erop
              plot( e6 , add = TRUE )
but I got this

         Error: invalid graphics state
        Error: invalid graphics state



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