Mask() in raster package only returns RasterLayers not RasterStack or RasterBrick

Dear members,

I am very happily using the raster package but here is a problem I can't 
solve.

When I use the mask function on  a RasterBrick or RasterStack:

raster_mask <- mask( raster_stack, SpatialPolygonDataFrame)

raster returns a RasterLayer and gives me a warning:

"In .rasterFromRasterFile(grdfile, band = band, objecttype) : size of 
values file does not match the number of cells (given the data type)"

When I use mask on a single layer from a brick or stack (e.g. mask( 
RasterStack[1], SpatialPolygon*) ) it returns a correctly masked 
rasterlayer.

I am trying to mask with a SpatialPolygonDataFrame, read in with 
readShapePoly of the maptools package.
R version 2.13.0 (2011-04-13)
Platform: x86_64-pc-mingw32/x64 (64-bit)


I am sure I missed something simple and I hope someone would point it 
out to me.

Thanks,
Robert