Error in compilation during macroexpansion of case form

[Paul F. Dietz]

(lambda (a)
  (case a
    ((#\i #\r) a)
    ((#\9) 0)
    ((#\6 #\H) 0)
    ((0 #\^ #\z) 0)
    ((#\E) 0)
    ((1 3 4 #\a) a)
    (t a)))

==>

; in: LAMBDA (CL-TEST::A)
; (CASE CL-TEST::A
; ((#\i #\r) CL-TEST::A)
; ((#\9) 0)
; ((#\6 #\H) 0)
; ((0 #\^ #\z) 0)
; ((#\E) 0)
; ((1 3 4 #\a) CL-TEST::A)
; (T CL-TEST::A))
;
; caught ERROR:
; during macroexpansion of (CASE CL-TEST::A ((#\LATIN_SMALL_LETTER_I #\LATIN_SMALL_LETTER_R) CL-TEST::A) ((#\DIGIT_NINE) 0) ((#\DIGIT_SIX #\LATIN_CAPITAL_LETTER_H) 0) ((0 #\CIRCUMFLEX_ACCENT #\LATIN_SMALL_LETTER_Z) 0) ((#\LATIN_CAPITAL_LETTER_E) 0) ((1 3 4 #\LATIN_SMALL_LETTER_A) CL-TEST::A) (T CL-TEST::A)). Use *BREAK-ON-SIGNALS* to intercept.
; The value NIL is not of type STRING when binding STRING

; (LAMBDA (CL-TEST::A)
; (CASE CL-TEST::A
; ((#\i #\r) CL-TEST::A)
; ((#\9) 0)
; ((#\6 #\H) 0)
; ((0 #\^ #\z) 0)
; ((#\E) 0)
; ((1 3 4 #\a) CL-TEST::A)
; (T CL-TEST::A)))
;
; caught STYLE-WARNING:
; The variable A is defined but never used.
;
; compilation unit finished
; caught 1 ERROR condition
; caught 1 STYLE-WARNING condition
#<FUNCTION (LAMBDA (A)) {554927EB}>

x86-64, "2.4.2.187-d15363cef"