[BioC] Problem with Expresso and RMA

[Monnie McGee]

Dear BioC Users,

I am analyzing data from an AffyBatch object (that I've called  
"allarray") with the following features:

 > print(allarray)
AffyBatch object
size of arrays=1164x1164 features (63515 kb)
cdf=HG-U133_Plus_2 (54675 affyids)
number of samples=6
number of genes=54675
annotation=hgu133plus2

Note that there are no replicates in this array.  In other words, the 6  
samples are 6 different arrays with 6 different RNA treatments.  I'm  
trying to understand which genes are differentially expressed among the  
6 arrays.

I wanted to use RMA normalization, so I typed rma(allarray) and  
received the following error
 > eset1 <- rma(allarray)
Background correcting
Normalizing
Calculating Expression
Error in sigToEnv(signature, fdef) : Trying to get slot "signature"  
from an object of a basic class ("NULL") with no slots

I tried to do the same thing using expresso and received the same  
error.  Here's what I typed and the response of bioconductor:
 > eset1  
<- 
expresso(allarray,bgcorrect.method="rma",normalize.method="quantiles",pm 
correct.method="pmonly",summary.method="medianpolish")
background correction: rma
normalization: quantiles
PM/MM correction : pmonly
expression values: mas
background correcting...done.
normalizing...Error in sigToEnv(signature, fdef) : Trying to get slot  
"signature" from an object of a basic class ("NULL") with no slots

I read an archived message with said that it was not a good idea to use  
median polish without replicated arrays, and that it would be better to  
use rlm.  However, I can't seem to get an eset so that I can use rlm.

The funny thing is, just last week, expresso was working fine.  I was  
able to obtain an eset with no errors, using the same AffyBatch with  
the same choices for all the methods.

I would appreciate some help.

Thanks,
Monnie