Random unique integers - problem

I grabbed this code from an old posting and converted it to 4.0.  It does what it is supposed to do - generate random unique integers.  But there is a problem - if I request a dataset longer than the max value it never finishes - just crunches for ever. (in Prime or 15) I can't figure out why it would do that.  Anyone see the fix? (or have another tool that generates unique integers?)