spatstat error: owin & "im" object

Hello SIG-GEO community,

I followed the example for the Extract.im function to "subset" a object of
class "im" to the extent of an owin.

# make up an image
  X <- setcov(unit.square())
  plot <http://inside-r.org/r-doc/graphics/plot>(X)

  # a rectangular subset
  W <- owin(c <http://inside-r.org/r-doc/base/c>(0,0.5),c
<http://inside-r.org/r-doc/base/c>(0.2,0.8))
  Y <- X[W]
  plot <http://inside-r.org/r-doc/graphics/plot>(Y)


However, when I applied to my data, it does not properly work, the output
object is not a 'im'

Here the code:

# 1. Import tiff format file. It is a raster file, spatial resolution 30 x
30 m,  ny=2667, nx=700. Pixel value ranges: 1 to 11, each one represent a
category.

tn<-as(readGDAL("..../rasters/tnn.tif"),"im")

class(tn)

[1] "im"

# 2. Subset to owin extent:

class(W)

[1] "owin"

# note: *W* was created from a irregular polygonal shapefile, and it is the
same file used to define the owin for the point pattern

tn2<-tn[W]
class(tn2)
[1] "integer"


The same problem occur with all the tiff-format imported grids, (e.g. DEM)


Has anyone experienced this problem before? Does anyone has a suggestion? I
will appreciate your input.