defmethod specializer on string match

Question

For numbers, I can specialise on an exact value within a defmethod this works:

(defgeneric person (attribute)
  (:documentation "attributes of a person"))

(defmethod person ((a number))
  (let ((sum (+ a 10)))
    (format t "The sum of 10 and ~a is ~a" a sum)))

(defmethod person ((a (eql 10)))
    (format t "TENNNNNN!!!"))

> (person 11) ;; The sum of 10 and 11 is 21
> (person 10) ;; TENNNNN!!

How can I accomplish the same thing for a string match?

I tried eq, equal and string= but I get an error that says not a valid parameter specializer name?