R CMD check fails with dev dplyr

[Hadley Wickham]

checking examples ... ERROR
Running examples in ‘statar-Ex.R’ failed
The error most likely occurred in:

> base::assign(".ptime", proc.time(), pos = "CheckExEnv")
> ### Name: duplicates
> ### Title: returns duplicated rows
> ### Aliases: duplicates duplicates_
> 
> ### ** Examples
> 
> library(data.table)
> DT <- data.table(a = rep(1:2, each = 3), b = 1:6)
> duplicates(DT, by = "a")
2 groups have duplicates
   N a b
1: 3 1 1
2: 3 1 2
3: 3 1 3
4: 3 2 4
5: 3 2 5
6: 3 2 6
> duplicates(DT, by = list(a,b))
Error: All select() inputs must resolve to integer column positions.
The following do not:

Could you please take a look? Thanks!