The value NIL is not of type SB-KERNEL:CTYPE when binding TYPE

[Paul F. Dietz]

(lambda (s1 s2 p x y)
  (let ((f (when p #'string-equal)))
    (when f
      (funcall f s1 s2 x y)
      )))

==>

The value
  NIL
is not of type
  SB-KERNEL:CTYPE
when binding TYPE
[...]
  0: (SB-C::VALIDATE-CALL-TYPE #<SB-C::COMBINATION :FUN #<SB-C::REF :%SOURCE-NAME SB-C::THING :LEAF #<SB-C::LAMBDA-VAR :%SOURCE-NAME F :TYPE #<SB-KERNEL:UNION-TYPE #> {1018739D23}> {10187454B3}> :ARGS (#<S..
  1: (SB-C::IR1-OPTIMIZE-COMBINATION #<SB-C::COMBINATION :FUN #<SB-C::REF :%SOURCE-NAME SB-C::THING :LEAF #<SB-C::LAMBDA-VAR :%SOURCE-NAME F :TYPE #<SB-KERNEL:UNION-TYPE #> {1018739D23}> {10187454B3}> :ARG..
  2: (SB-C::IR1-OPTIMIZE-BLOCK #<SB-C::CBLOCK NIL :START c3 {1018745353}>)
  3: (SB-C::IR1-OPTIMIZE #<SB-C:COMPONENT :NAME (LAMBDA (S1 S2 P X Y)) :REANALYZE T {101873E663}> NIL)
  4: (SB-C::IR1-OPTIMIZE-UNTIL-DONE #<SB-C:COMPONENT :NAME (LAMBDA (S1 S2 P X Y)) :REANALYZE T {101873E663}>)
  5: (SB-C::IR1-PHASES #<SB-C:COMPONENT :NAME (LAMBDA (S1 S2 P X Y)) :REANALYZE T {101873E663}>)
  6: (SB-C::COMPILE-COMPONENT #<SB-C:COMPONENT :NAME (LAMBDA (S1 S2 P X Y)) :REANALYZE T {101873E663}>)
[...]