0. emacs -Q
1. M-x load-library RET map RET
2. (let ((l (list (cons 'a 1))))
     (setf (map-elt l "a" nil #'string=) 2)
     l)
   C-j

This correctly gives ((a . 2)).

3. (let ((l (list (cons 'a 1))))
     (cl-incf (map-elt l "a" nil #'string=))
     l)
   C-j

This gives the following backtrace: