[BioC] lumi: error when plotting CV for lumi expression set

[Sebastien Gerega]

I am getting the following error when trying to plot the coefficient of 
variance for a lumi expression et.
plot(x.lumi, what='cv')
Error in as.double(y) :
  cannot coerce type 'S4' to vector of type 'double'

I have not had any problems creating the other QC plots that are 
recommended in the vignette.
What could be causing this problem, or how can I diagnose it?
thanks,
Sebastien