Exactly. After I posted, I tested it on v1.9.6 and on a mid-ish v1.9.7 build from last year -- all crash. I am glad I was able to isolate this issue. I think I have seen it before a while a few years ago. Can I just say that you and Arun are my heroes, and about 95% of the reason I am even in this field. If I didn't have data.table, I wouldn't have gotten into R. So if you guys need any help at all just tell me. O, and my example might sound like a strange use case, but it is actually quite practical. It is only one line of code, but it is extremely powerful.