r - Lines on reprojected sp objects with Mollweide projection

I'm having an odd problem such that when I reproject maps to a Mollweide projection in R using sp, I get additional lines stretching across the globe. What is going on here, and do you have any suggestions to fix it. Here's a reproductible example:

library(sp)
library(maps)
library(maptools)

worldmap <- map("world", plot=F)

plot(worldmap, type="l)

So far, so good. I get

So now I reproject. This is a CRS I'm using to be compatible with some other projects. Note how world-spanning lines then crop up.

worldmapLines <- map2SpatialLines(worldmap,
                                  proj4string=CRS("+proj=longlat +datum=WGS84"))

worldM <- spTransform(worldmapLines,  
                      CRS("+proj=moll +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84 +units=m +no_defs"))


plot(worldM)