1.9.7 rolling join not working correctly

[rrichmond]

I just updated to the latest revision of 1.9.7 and it looks like rolling joins aren't working correctly. Here's an example on 1.9.7:

library(data.table)
library(zoo)
library(lubridate)

mydates <- data.table(yearmon=as.yearmon(2000+2/12+seq(0,1,by=3/12)))

othervals <- data.table(date=mdy("12301999")+months(seq(0,24,by=6)))
othervals[,yearmon:=as.yearmon(date)]

setkey(othervals,yearmon)
setkey(mydates,yearmon)

othervals[mydates,roll=TRUE]
othervals[mydates,roll=1/12]

> othervals[mydates,roll=TRUE]
         date  yearmon
1: 1999-12-30 Mar 2000
2: 2000-06-30 Jun 2000
3: 2000-06-30 Sep 2000
4: 2000-12-30 Dec 2000
5: 2000-12-30 Mar 2001
> othervals[mydates,roll=1/12]
         date  yearmon
1:       <NA> Mar 2000
2: 2000-06-30 Jun 2000
3: 2000-06-30 Sep 2000
4: 2000-12-30 Dec 2000
5:       <NA> Mar 2001

Whereas on 1.9.6 the correct output is:

> othervals[mydates,roll=TRUE]
         date  yearmon
1: 1999-12-30 Mar 2000
2: 2000-06-30 Jun 2000
3: 2000-06-30 Sep 2000
4: 2000-12-30 Dec 2000
5: 2000-12-30 Mar 2001
> othervals[mydates,roll=1/12]
         date  yearmon
1:       <NA> Mar 2000
2: 2000-06-30 Jun 2000
3:       <NA> Sep 2000
4: 2000-12-30 Dec 2000
5:       <NA> Mar 2001
> 

On 1.9.6 the index difference is being correctly calculated whereas on 1.9.7 it is not.