Problem with format in overlay()

Hi!

I do:

bc <- overlay(sN, sSM, fun=fun1,filename="testoverlay.tif",overwrite=TRUE)

but if I specify the format:

bc <- overlay(sN, sSM, fun=fun1,filename="testoverlay.tif",overwrite=TRUE,format='GTiff',datatype='INT1U')

Error in .rasterObjectFromFile(x, band = band, objecttype = "RasterLayer",  :
  file: /media/FREECOM_HDD/VGT/SM/GTiff does not exist
Calls: overlay ... raster -> raster -> .local -> .rasterObjectFromFile

writeRaster(bc,
filename="bc.tif",overwrite=TRUE,format="GTiff",datatype='INT2U')
works fine, therefore the error above is not big issue.

Thanks

Agus