1) Version 2.2, LM performs as I stated. I don't have 2.4. Personally, I prefer this performance. 2) CLtL, p.281, "if NIL appears in the a-list in place of a pair ... ASSOC will ignore the NIL in the a-list and continue to search for an actual pair (cons) whose car is NIL." If 2.4 returns NIL, then it is a bug.