raster: cannot extract object from list

A function of mine outputs several raster objects in a list "l":

class(l$lisaobsv)

[1] "RasterLayer"
attr(,"package")
[1] "raster"

But by some reason I get this weird error:

a = l$lisaobsv

Error in slot(x, slotname) :
?no slot of name "z" for this object of class "RasterLayer"
Calls: <Anonymous> -> lapply -> FUN -> slot -> .Call
Warning message:
In .rk.get.structure.global("a") : failure to get object a

Is this a bug or is there an special way to extract raster objects from a list?

Thanks

Agus

sessionInfo()

R version 2.13.0 (2011-04-13)
Platform: x86_64-pc-linux-gnu (64-bit)

locale:
?[1] LC_CTYPE=en_US.UTF-8 ? ? ? ? ?LC_NUMERIC=C
?[3] LC_TIME=en_US.UTF-8 ? ? ? ? ? LC_COLLATE=en_US.UTF-8
?[5] LC_MONETARY=en_US.UTF-8 ? ? ? LC_MESSAGES=en_US.UTF-8
?[7] LC_PAPER=en_US.UTF-8 ? ? ? ? ?LC_NAME=en_US.UTF-8
?[9] LC_ADDRESS=en_US.UTF-8 ? ? ? ?LC_TELEPHONE=en_US.UTF-8
[11] LC_MEASUREMENT=en_US.UTF-8 ? ?LC_IDENTIFICATION=en_US.UTF-8

attached base packages:
[1] grid ? ? ?stats ? ? graphics ?grDevices utils ? ? datasets ?methods
[8] base

other attached packages:
?[1] rgdal_0.6-33 ?geoR_1.6-27 ? plotrix_3.2-2 ggplot2_0.8.9 proto_0.3-8
?[6] reshape_0.8.4 plyr_1.4 ? ? ?raster_1.8-40 sp_0.9-76 ? ? rkward_0.5.6

loaded via a namespace (and not attached):
[1] lattice_0.19-26 tools_2.13.0