bug#34272: 26.1.91 M-x flyspell-mode

[Tak Kunihiro]

I cannot start flyspell-mode on GNU Emacs 26.1.91 (build 1, x86_64-w64-mingw32).
I can    start flyspell-mode on GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
I wonder if this is something to do with 26.1.91.

CMD> c:/msys64/mingw64/bin/hunspell.exe --version
@(#) International Ispell Version 3.2.06 (but really Hunspell 1.6.2)

GNU Emacs 26.1.91 (build 1, x86_64-w64-mingw32)
CMD> runemacs -Q

M-: (with-eval-after-load "ispell"
        (ispell-change-dictionary "en_US" t))
M-: (setq ispell-program-name "c:/msys64/mingw64/bin/hunspell.exe"
          ispell-local-dictionary "en_US"
          ispell-local-dictionary-alist
          '(("en_US" "[[:alpha:]]" "[^[:alpha:]]" "[']" nil ("-d" "en_US") nil 
utf-8)))
M-x flyspell-mode
  Error enabling Flyspell mode:
   (stringp nil)