[R] unbalanced design

[Damián Cirelli]

Hi all,
I'm new to R and have the following problem:
I have a 2 factor design (a has 2 levels, b has 3 levels). I have an 
object kidney.aov which is an aov(y ~ a*b), and when I ask for 
model.tables(kidney.avo, se=T) I get the following message along with 
the table of effects:

Design is unbalanced - use se.contrast() for se's

but the design is NOT unbalanced... each fator level combination has the 
same n

I' d appreciate any help.
Thanks.