x= data.table(id="aaa")
y=data.frame(id="bbb")
class(y) = c("tbl_df", "tbl", "data.frame")
x[!y, on="id"]
# Error in bmerge(i, x, leftcols, rightcols, roll, rollends, nomatch, mult, : # Internal error in forderReuseSorting: reuseSorting set to TRUE but DT is not a data.table. Please report to the data.table issues tracker.
No error on 1.15.4. No error if y is just plain data.frame.
It's the bmerge() code here that tries to call the new routine on the non-data.frame:
Activity