R help - Apr 2003 - nlme groupedData warning

I have a question about nlme (and yes, I have a copy of
"Mixed-Effects Models in S and S-Plus" on order :-)

I have a data frame, df, that I use to create a groupedData
object:
> df.K1 <- groupedData(K1 ~ tissue.ROI | scan, data=df )
If I add an outer grouping factor, I get a warning:
> df.K1 <- groupedData(K1 ~ tissue.ROI | scan, data=df, outer=~input.ROI )
Warning message: 
argument lengths differ in: split(x, f) 

Can anyone help me understand where this warning message is
coming from?  And what I might make of it?

Thanks, Mike

-- 
Michael A. Miller                               mmiller3 at iupui.edu
  Imaging Sciences, Department of Radiology, IU School of Medicine