[BioC] MA plots without bg correction (mArray)

[Foata,Francis,LAUSANNE,NRC/N&H]

Hello, I'm using the mArray package and I would like to make MA plots
WITHOUT background correction. I tested the following R code :

xma<-log2(maRf(christian.raw[,1])/maGf(christian.raw[,1]))
yma<-0.5*log2(maGf(christian.raw[,1])+maRf(christian.raw[,1]))
maPlot (christian.raw[,1], x=yma, y=yma, text.func = blanks.spots)

I obtained this error :
Error in strsplit(x, as.character(split), as.logical(extended)) : 
        non-character argument in strsplit()

Does anybody know how to handle this problem? 
I would really like to use the function maPlot because I would like to keep
the 'highlighted spots' on the graph(nonrealizable (?) thing with the
function plot)
 

Best regards,


Francis