Recipe to create the error:

- emacs -Q
- Create two buffers; I am calling them a and b: C-x b a, C-x b b
- Put something in each buffer to diff.. I put "abc" and "abcd"
- M-x ediff-buffers, and select those two buffers
- Hit 'n' key to go to the first line showing the difference.

You would then get:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  find-file-name-handler(nil file-local-copy)
  file-local-copy(nil)
  #[(file) "\301 !\206
  mapcar(#[(file) "\301 !\206
  ediff-exec-process("diff" #<buffer *ediff-fine-diff<2>*> synchronize ""
"/tmp/fineDiffA15751M4K" "/tmp/fineDiffB15751ZCR" nil)
  ediff-setup-fine-diff-regions("/tmp/fineDiffA15751M4K"
"/tmp/fineDiffB15751ZCR" nil 0)
  ediff-make-fine-diffs(0 noforce)
  ediff-install-fine-diff-if-necessary(0)
  ediff-next-difference(1)
  funcall-interactively(ediff-next-difference 1)
  call-interactively(ediff-next-difference nil nil)
  command-execute(ediff-next-difference)

[image: pasted1]

In GNU Emacs 26.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 2.24.23)
 of 2017-04-05 built on ...
Repository revision: f1d34d9136fbf1dc2cf58b5ba36311451f024956
Windowing system distributor 'The X.Org Foundation', version 11.0.60900000
System Description: Red Hat Enterprise Linux Workstation release 6.6
(Santiago)

Recent messages:
file-local-copy: Wrong type argument: stringp, nil
Debug on Error enabled globally
Debug on Error disabled globally
Debug on Error enabled globally
Computing differences between ediff15751Nk1 and ediff15751_tE ...
Buffer A: Processing difference region 0 of 1
Buffer B: Processing difference region 0 of 1
Processing difference regions ... done
Refining difference region 1 ...
Entering debugger...

Configured using:
 'configure --with-modules
 --prefix=/home/kmodi/usr_local/apps/6/emacs/master
 '--program-transform-name=s/^ctags$/ctags_emacs/'
 'CPPFLAGS=-fgnu89-inline -I/home/kmodi/usr_local/6/include
 -I/usr/include/freetype2 -I/usr/include' 'CFLAGS=-ggdb3 -O0'
 'CXXFLAGS=-ggdb3 -O0' 'LDFLAGS=-L/home/kmodi/usr_local/6/lib
 -L/home/kmodi/usr_local/6/lib64 -ggdb3'
 PKG_CONFIG_PATH=/home/kmodi/usr_local/6/lib/pkgconfig:/home/kmodi/usr_local/6/lib64/pkgconfig:/cad/adi/apps/gnu/linux/x86_64/6/lib/pkgconfig:/cad/adi/apps/gnu/linux/x86_64/6/lib64/pkgconfig:/usr/lib/pkgconfig:/usr/lib64/pkgconfig:/usr/share/pkgconfig:/lib/pkgconfig:/lib64/pkgconfig'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK2 X11 MODULES

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=none
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message puny seq byte-opt subr-x gv
bytecomp byte-compile cl-extra cconv dired dired-loaddefs format-spec
rfc822 mml mml-sec password-cache epa derived epg epg-config gnus-util
rmail rmail-loaddefs mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils help-mode easymenu debug cus-start
cus-load ediff-merg ediff-wind ediff-diff ediff-mult ediff-help
ediff-init cl-loaddefs pcase cl-lib ediff-util ediff time-date mule-util
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite charscript case-table epa-hook jka-cmpr-hook help
simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs button
faces cus-face macroexp files text-properties overlay sha1 md5 base64
format env code-pages mule custom widget hashtable-print-readable
backquote dbusbind inotify dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 114425 9298)
 (symbols 48 22242 1)
 (miscs 40 143 141)
 (strings 32 22533 4406)
 (string-bytes 1 740306)
 (vectors 16 16276)
 (vector-slots 8 506737 6409)
 (floats 8 72 54)
 (intervals 56 433 0)
 (buffers 976 25)
 (heap 1024 45428 673))

-- 

Kaushal Modi