如何在R中剪切带有多边形的WorldMap？

I have imported a world map dataset from www.GADM.org using the R package raster. I would like to clip it to a polygon I create to reduce the size of the map. I can retrieve the data and I can create the polygon no problem, but when I use the 'gIntersection' command I get an obscure error message.I have imported a world map dataset from www.GA