dplyr/summarize does not create a true data frame

I got an error when trying to extract a 1-column subset of a data frame (called "my.output") created by dplyr/summarize. The ncol() function says that my.output has 4 columns, but "my.output[4]" fails. Note that converting my.output using as.data.frame() makes for a happy ending.

Is this the intended behavior of dplyr?

Tx,
John

library(dplyr)

# set up data frame
rows = 100
repcnt = 50
sexes = c("Female", "Male")
heights = c("Med", "Short", "Tall")

frm = data.frame(

+   Id = paste("P", sprintf("%04d", 1:rows), sep=""),
+   Sex = sample(rep(sexes, repcnt), rows, replace=T),
+   Height = sample(rep(heights, repcnt), rows, replace=T),
+   V1 = round(runif(rows)*25, 2) + 50,
+   V2 = round(runif(rows)*1000, 2) + 50,
+   V3 = round(runif(rows)*350, 2) - 175
+ )

# use dplyr/summarize to create data frame
my.output = frm %>%

+   group_by(Sex, Height) %>%
+   summarize(V1sum=sum(V1), V2sum=sum(V2))

# work with columns in the output data frame
ncol(my.output)

[1] 4

my.output[1]

Source: local data frame [6 x 1]
Groups: Sex

     Sex
1 Female
2 Female
3 Female
4   Male
5   Male
6   Male

my.output[4]

Error in eval(expr, envir, enclos) : index out of bounds  ######## ERROR HERE

as.data.frame(my.output)[4]

V2sum
1 12427.97
2  8449.82
3  8610.97
4  7249.20
5 12616.91
6 10372.15