Error using rfe in caret package in R

Question

I am doing some exploratory data analysis in the Heritage Health Prize , and have come across a weird error using R's caret package. In the dataset, I've created a dataframe counting how many times a patient (by MemberID) has visited a specific primary care physician (denoted by a number) in year 1 insurance claims. Let's say there are 57 such primary care physicians, and so our dataframe looks like this:

> head(cbind(right.a[,1],right.a[,107:111]))
      MemberID 2448 91972 20893 30870 13281
1     24027523    0     0     0     0     0
2     98321677    2     0     0     3     0
3     33896267    0     1     0     0     1
4      5481382    0     0     2     0     0
5     69908341    0     0     0     0    17
6      8169352    0     0     0     0     0

As you can see, this is a very sparse matrix.

Our outcome here is the number of days they spent in hospital the following year in year 2 (DaysInHospital). I'm using year 2 insurance claims and year 3 days in hospital as a validation set.

I'm using rfe to do some recursive feature selection, and I'm getting the following error:

> subsets <- seq(12,57,by = 3)
> lmProfile <- rfe(right.a[,c(107:164)],
             log1p(right.a$DaysInHospital),
             right.b[,c(107:164)],
             log1p(right.b$DaysInHospital),
             sizes=subsets,
             metric="RMSE",
             maximize="FALSE",
             rfeControl=ctrl)
.
(snip)
.
External resampling iter:        10 
Computing importance
Fitting subset size:   58 
Computing importance
Computing importance
Error in y$pred : $ operator is invalid for atomic vectors

Am I missing something here?