When one has not set alloc.col to be sufficiently high explicitly and attempts to create more columns than max(100L,ncol(DT)+64L), the following error is printed:

Internal logical error. DT passed to assign has not been allocated enough column slots.

I did not know what this meant, but I found an answer on Stack Overflow. It would be helpful if the package would explicitly direct the user to help(truelength).

Also, for some reason I only encountered this issue with set(), but not :=.