match-string debugging problem

[Stephen Berman]

There seems to be something about match-string that I don't
understand.  Here is an example of the kind of code I'm working with:

(defvar mystring1 "+++++ ")
(defvar mystring2 " ~~~~~")
(defun mystring-list ()
  (interactive)
  (with-current-buffer (get-buffer-create "*test*")
    (switch-to-buffer "*test*")
    (dotimes (num 5)
      (insert mystring1 "test" (int-to-string (1+ num)) mystring2 "\n"))
    (goto-char (point-min))
    (let ((mystring-list ()))
      (while (re-search-forward
              (concat "^" (regexp-quote mystring1) "\\(.+\\)"
                      (regexp-quote mystring2) "$")
              (point-max) t)
        (setq mystring-list (append (list (match-string 1)) mystring-list)))
      (insert "\n")
      (setq mystring-list (reverse mystring-list))
      (dolist (elt mystring-list)
        (insert elt " ")))))

After evalling this code and typing `M-x mystring-list', buffer *test*
consists of these lines:

+++++ test1 ~~~~~
+++++ test2 ~~~~~
+++++ test3 ~~~~~
+++++ test4 ~~~~~
+++++ test5 ~~~~~
test1 test2 test3 test4 test5 

The last line indicates that match-string correctly matches the
strings that build mystring-list.  But when I step through the code
with edebug, match-string always returns nil and a wrong-type-argument
error is raised at the insert (since nil is not char-or-string-p).
(Edebug isn't the problem: evalling first the regexp search code in
*test* and then (match-string 1) also returns nil.)  Because of this
I'm having a hard time debugging other code that uses match-string.
Can someone explain what's going on?

Steve Berman