[R-sig-Geo] gIntersection gives error (may not contain geometry collections)

[Graham Bond McNamara]

I am trying to use gIntersection like so:

gIntersection(mypolys.spdf, mylines.sldf, byid=c(TRUE,TRUE))

This intersection between the polygons in the SpatialPolygonsDataFrame
and the lines in the SpatialLinesDataFrame should result in a new
SpatialLines object. I have used this before and this works.

However, on my new dataset, I get the following error:

output subgeometry 3195, row.name: 24 3759
subsubgeometry 0: Point
subsubgeometry 1: LineString
Error in RGEOSBinTopoFunc(spgeom1, spgeom2, byid, id, drop_not_poly,
"rgeos_intersection") :
  Geometry collections may not contain other geometry collections

I have inspected mypolys.spdf and my lines.sldf, and they are not
geometry collections, but just the spdf and sldf. So I don't know
where the "Point" and "LineString" are coming from (what is a
LineString anyway?).

The culprit seems to be the combination of  poly 24 and line 3759.
There is nothing wrong with it per se, but it's a line that exactly
overlaps the polygon border. I thought perhaps that was the problem?

I am not allowed to share my actual data, but it looks like this:

Sr1 = Polygon(cbind(c(1,2,2,4,4,1,1),c(1,1,3,3,5,5,1)))
Sr2 = Polygon(cbind(c(2,4,4,2,2),c(1,1,3,3,1)))
Srs1 = Polygons(list(Sr1), "1")
Srs2 = Polygons(list(Sr2), "2")
SpP = SpatialPolygons(list(Srs1,Srs2), 1:2)
plot(SpP) # example neighborhoods

l1 = cbind(c(2,2),c(2,4))
Sl1 = Line(l1)
S1 = Lines(list(Sl1), ID="1")
Sl = SpatialLines(list(S1))
plot(Sl, add=TRUE, col="green", lwd=3)

gIntersection works:

gInt <- gIntersection(SpP, Sl, byid=c(TRUE, TRUE))

The line hugging the border intersects both poly 1 and poly 2, so this
line is split up and reproduced twice.

In summary: gIntersection works even when a line is exactly on the
poly's border. This means I am no closer to understanding my error
message. Can anyone help?

Cheers,
Graham