[BioC] pairwise.comparison invalid object for slot "pData"

[David Ruau]

Dear List,

I have a problem using the pairwise.comparison function in the  
simpleaffy package.
 > x.rma <- call.exprs(da, "rma")
Background correcting
Normalizing
Calculating Expression
 > class(x.rma)
[1] "ExpressionSet"
attr(,"package")
[1] "Biobase"
 > pData(x.rma)
     population
KO          KO
het        het
 > results <- pairwise.comparison(x.rma, "population", spots=da)
Error in validObject(.Object) :
   invalid class "PairComp" object: invalid object for slot "pData" in  
class "PairComp": got class "factor", should be or extend class  
"data.frame"

I already posted this error a while a go but without obtaining any  
answer...

If anybody has experienced the same problem and managed to solved it,  
I would appreciate that he or she share his solution...

Best,
David
---
 > sessionInfo()
R version 2.7.1 (2008-06-23)
i386-apple-darwin9.4.0

locale:
en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/C

attached base packages:
[1] splines   tools     stats     graphics  grDevices datasets   
utils     methods   base

other attached packages:
  [1] mouse4302cdf_2.2.0   simpleaffy_2.16.0    gcrma_2.12.1          
matchprobes_1.12.0   genefilter_1.20.0
  [6] survival_2.34-1      affy_1.18.2          preprocessCore_1.2.1  
affyio_1.8.1         Biobase_2.0.1

loaded via a namespace (and not attached):
[1] AnnotationDbi_1.2.2 DBI_0.2-4           RSQLite_0.6-9        
annotate_1.18.0     tcltk_2.7.1