Uncaught compiler error calling ASSOC with an improper list for the second argument

[Paul F. Dietz]

(lambda (x) (assoc x '((a . 1) . b)))

===>

The value
  B
is not of type
  LIST
   [Condition of type TYPE-ERROR]
[...]
  0: ((FLET SB-IMPL::FAST-NTHCDR :IN NTHCDR) 128 ((A . 1) . B))
  1: (SB-C::TRANSFORM-LIST-ITEM-SEEK ASSOC #<SB-C::LVAR 1 {10092054C3}> #<SB-C::LVAR 2 {1009205603}> NIL NIL NIL #<SB-C::COMBINATION :FUN #<SB-C::REF :LEAF #<SB-C::GLOBAL-VAR :%SOURCE-NAME ASSOC :TYPE #1=..
  2: ((SB-C:DEFTRANSFORM ASSOC) #<SB-C::COMBINATION :FUN #<SB-C::REF :LEAF #<SB-C::GLOBAL-VAR :%SOURCE-NAME ASSOC :TYPE #1=#<SB-KERNEL:FUN-TYPE #> :DEFINED-TYPE #1# :WHERE-FROM :DECLARED :KIND :GLOBAL-FUN..
[...]