Evaluating in the scratch buffer:

(setq ispell-program-name "hunspell")
"hunspell"

(require 'ispell)
ispell

(ispell-find-hunspell-dictionaries)

Causes this error:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  string-match("," nil 0)
  split-string(nil "," t)
  (let ((dict-list (split-string dict-key "," t)) (first-p t) (dict-arg "") otherchars-list) (let ((--dolist-tail-- dict-list)) (while --dolist-tail-- (let ((dict-key (car --dolist-tail--))) (let ((affix-file (car (cdr (assoc dict-key ispell-hunspell-dict-paths-alist))))) (if affix-file nil (error "ispell-phaf: No matching entry for %s in `ispell-hunspell-dict-paths-alist'.\n" dict-key)) (if (and first-p (not (file-exists-p affix-file))) (error "ispell-phaf: File \"%s\" not found.\n" affix-file)) (and first-p (setq first-p nil)) (let ((dict-name (file-name-sans-extension (file-name-nondirectory affix-file))) otherchars-string) (let ((temp-buffer (generate-new-buffer " *temp*"))) (save-current-buffer (set-buffer temp-buffer) (unwind-protect (progn (insert-file-contents affix-file) (setq otherchars-string (save-excursion (goto-char (point-min)) (if (search-forward-regexp "^WORDCHARS +" nil t) (buffer-substring (point) (progn (end-of-line) (point)))))) (if otherchars-string (let* ((otherchars-string (substring otherchars-string 0 (string-match " +" otherchars-string))) (chars-list (append otherchars-string nil))) (setq chars-list (delq 32 chars-list)) (let ((--dolist-tail-- chars-list)) (while --dolist-tail-- (let ((ch (car --dolist-tail--))) (setq otherchars-list (if (member ch otherchars-list) otherchars-list (cons ch otherchars-list))) (setq --dolist-tail-- (cdr --dolist-tail--)))))))) (and (buffer-name temp-buffer) (kill-buffer temp-buffer))))) (setq dict-arg (concat dict-arg (if (> (length dict-arg) 0) ",") dict-name)))) (setq --dolist-tail-- (cdr --dolist-tail--))))) (list dict-key "[[:alpha:]]" "[^[:alpha:]]" (if otherchars-list (regexp-opt (mapcar (function char-to-string) otherchars-list)) "") t (list "-d" dict-arg) nil 'utf-8))
  ispell-parse-hunspell-affix-file(nil)
  (setq hunspell-default-dict-entry (ispell-parse-hunspell-affix-file hunspell-default-dict))
  (let ((hunspell-found-dicts (split-string (let ((temp-buffer (generate-new-buffer " *temp*"))) (save-current-buffer (set-buffer temp-buffer) (unwind-protect (progn (ispell-call-process ispell-program-name null-device t nil "-D") (buffer-string)) (and (buffer-name temp-buffer) (kill-buffer temp-buffer))))) "[\n\015]+" t)) hunspell-default-dict hunspell-default-dict-entry hunspell-multi-dict) (let ((--dolist-tail-- hunspell-found-dicts)) (while --dolist-tail-- (let ((dict (car --dolist-tail--))) (let* ((full-name (file-name-nondirectory dict)) (basename (file-name-sans-extension full-name)) (affix-file (concat dict ".aff"))) (if (string-match "\\.aff$" dict) (progn (if hunspell-default-dict (setq hunspell-multi-dict (concat (or hunspell-multi-dict (car hunspell-default-dict)) "," basename)) (setq affix-file dict) (setq hunspell-default-dict (list basename affix-file))) (ispell-print-if-debug "++ ispell-fhd: default dict-entry:%s name:%s basename:%s\n" dict full-name basename)) (if (and (not (assoc basename ispell-hunspell-dict-paths-alist)) (file-exists-p affix-file)) (let ((affix-file (expand-file-name affix-file))) (ispell-print-if-debug "++ ispell-fhd: dict-entry:%s name:%s basename:%s affix-file:%s\n" dict full-name basename affix-file) (setq ispell-hunspell-dict-paths-alist (let* ((va (list basename affix-file))) (if (member va ispell-hunspell-dict-paths-alist) ispell-hunspell-dict-paths-alist (cons va ispell-hunspell-dict-paths-alist))))) (ispell-print-if-debug "-- ispell-fhd: Skipping entry: %s\n" dict)))) (setq --dolist-tail-- (cdr --dolist-tail--))))) (let (newlist) (let ((--dolist-tail-- ispell-dicts-name2locale-equivs-alist)) (while --dolist-tail-- (let ((dict (car --dolist-tail--))) (if (assoc (car dict) ispell-hunspell-dict-paths-alist) (ispell-print-if-debug "-- ispell-fhd: Excluding %s alias.  Standalone dict found.\n" (car dict)) (setq newlist (if (member dict newlist) newlist (cons dict newlist)))) (setq --dolist-tail-- (cdr --dolist-tail--))))) (setq ispell-dicts-name2locale-equivs-alist newlist)) (let ((--dolist-tail-- ispell-dicts-name2locale-equivs-alist)) (while --dolist-tail-- (let ((dict-equiv (car --dolist-tail--))) (let ((dict-equiv-key (car dict-equiv)) (dict-equiv-value (car (cdr dict-equiv))) (exclude-aliases (list "esperanto-tex" "francais7" "francais-tex" "norsk7-tex"))) (if (and (assoc dict-equiv-value ispell-hunspell-dict-paths-alist) (not (assoc dict-equiv-key ispell-hunspell-dict-paths-alist)) (not (member dict-equiv-key exclude-aliases))) (let ((affix-file (car (cdr (assoc dict-equiv-value ispell-hunspell-dict-paths-alist))))) (ispell-print-if-debug "++ ispell-fhd: Adding alias %s -> %s.\n" dict-equiv-key affix-file) (setq ispell-hunspell-dict-paths-alist (let* ((va (list dict-equiv-key affix-file))) (if (member va ispell-hunspell-dict-paths-alist) ispell-hunspell-dict-paths-alist (cons va ispell-hunspell-dict-paths-alist))))))) (setq --dolist-tail-- (cdr --dolist-tail--))))) (setq hunspell-default-dict (or ispell-dictionary hunspell-multi-dict (car hunspell-default-dict))) (setq hunspell-default-dict-entry (ispell-parse-hunspell-affix-file hunspell-default-dict)) (setq ispell-hunspell-dictionary-alist (list (cons nil (cdr hunspell-default-dict-entry)))) (let ((--dolist-tail-- (mapcar (function car) ispell-hunspell-dict-paths-alist))) (while --dolist-tail-- (let ((dict (car --dolist-tail--))) (setq ispell-hunspell-dictionary-alist (let* ((va (if (string= dict hunspell-default-dict) hunspell-default-dict-entry (list dict)))) (if (member va ispell-hunspell-dictionary-alist) ispell-hunspell-dictionary-alist (cons va ispell-hunspell-dictionary-alist)))) (setq --dolist-tail-- (cdr --dolist-tail--))))))
  ispell-find-hunspell-dictionaries()
  eval((ispell-find-hunspell-dictionaries) nil)
  elisp--eval-last-sexp(t)
  eval-last-sexp(t)
  eval-print-last-sexp(nil)
  funcall-interactively(eval-print-last-sexp nil)
  call-interactively(eval-print-last-sexp nil nil)
  command-execute(eval-print-last-sexp)


The output of hunspell -D is:

SEARCH PATH:
.::/home/gregor/.config/libreoffice/4/user/wordbook::/usr/share/hunspell:/usr/share/myspell:/usr/share/myspell/dicts:/Library/Spelling:/home/gregor/.openoffice.org/3/user/wordbook:/home/gregor/.openoffice.org2/user/wordbook:/home/gregor/.openoffice.org2.0/user/wordbook:/home/gregor/Library/Spelling:/opt/openoffice.org/basis3.0/share/dict/ooo:/usr/lib/openoffice.org/basis3.0/share/dict/ooo:/opt/openoffice.org2.4/share/dict/ooo:/usr/lib/openoffice.org2.4/share/dict/ooo:/opt/openoffice.org2.3/share/dict/ooo:/usr/lib/openoffice.org2.3/share/dict/ooo:/opt/openoffice.org2.2/share/dict/ooo:/usr/lib/openoffice.org2.2/share/dict/ooo:/opt/openoffice.org2.1/share/dict/ooo:/usr/lib/openoffice.org2.1/share/dict/ooo:/opt/openoffice.org2.0/share/dict/ooo:/usr/lib/openoffice.org2.0/share/dict/ooo
AVAILABLE DICTIONARIES (path is not mandatory for -d option):
/home/gregor/.config/libreoffice/4/user/wordbook/standard
/usr/share/hunspell/en_PH
/usr/share/hunspell/en_US-large
/usr/share/hunspell/en_US
/usr/share/myspell/dicts/en_PH
/usr/share/myspell/dicts/en_US-large
/usr/share/myspell/dicts/en_US
/home/gregor/Library/Spelling/en_GB
/home/gregor/Library/Spelling/sl_SI


Which causes the error since none of the listed dictionary files ends in
.aff, which causes the hunspell-default-dict to be nil on line 1179 in
ispell.el, which causes the error in the next sexp. The consequences of
this error are that spell checking does not work with hunspell and it is
not possible to set a dictionary via ispell-change-dictionary.

The quick and dirty fix of changing the line to

(setq hunspell-default-dict (or "british"
                                 hunspell-multi-dict
                                 (car hunspell-default-dict)))

(added "british" to the or) the problem goes away, and spell checking works

as expected.

hunspell version:

$ hunspell --version
@(#) International Ispell Version 3.2.06 (but really Hunspell 1.7.0)


Copyright (C) 2002-2014 László Németh. License: MPL/GPL/LGPL.

Based on OpenOffice.org's Myspell library.
Myspell's copyright (C) Kevin Hendricks, 2001-2002, License: BSD.

This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE,
to the extent permitted by law.

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2018-07-05 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
Recent messages:
Checking 105 files in /usr/share/emacs/26.1/lisp/obsolete...
Checking for load-path shadows...done
Mark set [2 times]
Auto-saving...done
Mark set
Sending...
Mark set [2 times]
Sending via mail...
Sending...done
Making completion list...

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
 -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
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ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11 MODULES THREADS LIBSYSTEMD LCMS2

Important settings:
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  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
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  eldoc-mode: t
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  auto-compression-mode: t
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  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(mailalias mailclient browse-url qp cus-edit cus-start cus-load wid-edit
pp shadow sort mail-extr emacsbug message rmc puny seq dired
dired-loaddefs format-spec rfc822 mml mml-sec password-cache epa derived
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mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils help-mode
easymenu cl-print byte-opt bytecomp byte-compile cconv debug ispell
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time-date mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
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dnd fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode elisp-mode lisp-mode prog-mode register page menu-bar
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syntax facemenu font-core term/tty-colors frame cl-generic cham georgian
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x-toolkit x multi-tty make-network-process emacs)

Memory information:
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 (symbols 48 22905 5)
 (miscs 40 134 517)
 (strings 32 35096 3840)
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