[unexpected] join operations on two data.tables yields a data.frame

[ctbrown]

Joining two data.table objects yield a data.frame.

 > class(milestone)
 [1] "data.table" "data.frame"
 > class(event)
[1] "data.table" "data.frame"
> class( inner_join( milestone, event ) )
Joining by: "parent_id"
[1] "data.frame"

I would expect that since both inputs are data.tables, the result would also be a data.table. It is probably a pretty is fix using is.data.table and setDT.