[R-sig-Geo] strip in spplot do not show

[Jaime R. Garcia Marquez]

Dear list users,

Surely, a very simple question but I did not manage to 
solve it after a while trying.

I want to use spplot to plot one variable of a 
SpatialGridDataFrame BUT I want the normal lattice 'strip' 
to appear in the plot.

data(meuse.grid)
coordinates(meuse.grid) <- c("x","y")
gridded(meuse.grid) <- TRUE
fullgrid(meuse.grid) <- TRUE

if I plot two variables

spplot(meuse.grid, zcol=c(1,2))

then the strip of each panel appears with the name of the 
variable, in this case 'part.a' and 'part.b'.

But if I plot just one variable the strip does not appear.

spplot(meuse.grid, zcol=1)

Not even when I specify the 'strip' argument

spplot(meuse.grid, zcol=1, strip=TRUE, names.attr="please 
show up")

spplot(meuse.grid, zcol=1, strip= 
strip.custom(var.names="Please show up"))

What am I missing???

Any help or suggestion is appreciated.

Best,

Jaime R.