Another spurious compilation diagnostic?

Greetings again, list readers:

It seems that every time I compile something like:

   (list-length some-random-list)

I get a compiler warning:

;   (LIST-LENGTH SOME-RANDOM-LIST)
; --> BLOCK DO BLOCK LET TAGBODY WHEN COND IF PROGN RETURN
; ==>
;   (RETURN-FROM NIL NIL)
; Warning: This is not a (VALUES &OPTIONAL REAL &REST T):
;   NIL
; ; [Last message occurs 2 times]

Any ideas?