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Hello,
according to variable 'ispell-dictionary-base-alist' defined in
'ispell.el' if 'ispell-dictionary' is nil then it is considered as
default value for dictionary. Unfortunately, when hunspell is used as
ispell program function 'ispell-parse-hunspell-affix-file' fails because
it doesn't expect nil as argument.

I propose small and simple patch. I checked it starting emacs as:
  emacs -Q
and evaluating in *scratch*:
  (setq ispell-program-name "hunspell")
  (setq ispell-really-hunspell t)
after that I run 'M-$' while point is on first 's' of
  (message)

In minibuffer I see
  MESSAGE is correct

Without this patch the same steps lead to message
  ispell-phaf: No matching entry for nil.