[BioC] fRMA error when using exprs()

[Sylvain Brohée]

Dear all,

I am quite new to the "affymetrix normalization data world" and until now, I 
was always normalizing my data using the good old RMA method.

People in the wet lab I am collaborating with asked me to renormalize a set of 
data coming from different labs and I discovered the fRMA method which is 
certainly more suitable in that case.

I am working on the mouse so I used the mouse4302frmavecs package.

This is my script :

rawdata <- ReadAffy(filenames = cel.files)
eset.frma <- frma(rawdata, summarize = 'batch');

When I run this, everything seems OK (about 5-6 to run) until I run this 
command :

eset.df <- exprs(eset.frma);

which produces the following error 

Error in function (classes, fdef, mtable)  : 
  unable to find an inherited method for function "exprs", for signature 
"PLMset"

and if I look at my eset.frma object, I get this 

Probe level linear model (PLMset) object
size of arrays=1002x1002
cdf=Mouse430_2 (45101 probeset ids)
number of samples=6
number of probesets=45101
number of chip level parameters for each probeset=6
annotation=mouse4302
PLMset settings
Creating function: 
Preprocessing
Background Correction=Error in if 
(object at model.description$preprocessing$background) { : 
  argument is of length zero


The only thing I'd like are the expression values of course. 

Any idea about the cause of the problem?

Many thanks to all of you,

Sylvain Brohée