[Pharo-project] primitive 65 failures

Why does primitive 65 (#next) include a test on the position vs. read length and sometimes fall back on indexing the collection? I have been creating my own stream for serial ports, and it was driving me nuts. After staring at the obvious (the primitive was failing sooner than I expected), I finally looked at Dolphin and saw the same test and fallback. Doing the analogous things in my code left it working a lot better than it had been. What gives? Are we perhaps suffering from a primitive that fails too easily and is running slow code as a result, or is there a "valid" reason for the primitive to fail? Bill