Error in "DataTable %>% group_by(V1) %>% filter(V2 == min(V2)) "

Case1: data.table

library(data.table)
library(dplyr)

df = data.table(V1 = c("A", "A", "B", "B"), V2 = c(1, 100, 2, 100))
df

##    V1  V2
## 1:  A   1
## 2:  A 100
## 3:  B   2
## 4:  B 100

## It does not work
df %>% 
  group_by(V1) %>% 
  filter(V2 == min(V2))

## Source: local data table [4 x 2]
## Groups: V1
## 
##   V1  V2
## 1  A   1
## 2  A 100
## 3  B   2
## 4  B 100

???

Case2: data.frame

It works.

df = data.frame(V1 = c("A", "A", "B", "B"), V2 = c(1, 100, 2, 100))
df %>% 
  group_by(V1) %>% 
  filter(V2 == min(V2))

## Source: local data frame [2 x 2]
## Groups: V1
## 
##   V1 V2
## 1  A  1
## 2  B  2

Experiment 1

df %>% as.data.table %>% 
  group_by(V1) %>% 
  filter(V2 == min(V2))

## Error: i has not evaluated to logical, integer or double

Experiment 2

By using iris data...

iris %>% as.data.table %>% 
  group_by(Species) %>% 
  filter(Sepal.Length == min(Sepal.Length))

## Source: local data table [3 x 5]
## Groups: Species
## 
##   Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
## 1          4.3         3.0          1.1         0.1     setosa
## 2          4.9         2.4          3.3         1.0 versicolor
## 3          4.9         2.5          4.5         1.7  virginica

It works. why??

sessionInfo()

## R version 3.1.0 (2014-04-10)
## Platform: x86_64-apple-darwin10.8.0 (64-bit)
## 
## locale:
## [1] ja_JP.UTF-8/ja_JP.UTF-8/ja_JP.UTF-8/C/ja_JP.UTF-8/ja_JP.UTF-8
## 
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
## 
## other attached packages:
## [1] dplyr_0.2        data.table_1.9.2 knitr_1.6       
## 
## loaded via a namespace (and not attached):
##  [1] assertthat_0.1 evaluate_0.5.5 formatR_1.0    magrittr_1.0.1
##  [5] parallel_3.1.0 plyr_1.8.1     Rcpp_0.11.2    reshape2_1.4  
##  [9] stringr_0.6.2  tools_3.1.0