autoload add-zsh-hook && provide a nicer error message if that fails due to old zsh version, or similar situations. Reset another variable in hook function; it can't hurt

zsh-syntax-highlighting.zsh

@@ -207,8 +207,12 @@ _zsh_highlight_load_highlighters "${ZSH_HIGHLIGHT_HIGHLIGHTERS_DIR:-${0:h}/highl
 _zsh_highlight_preexec_hook()
 {
   _ZSH_HIGHLIGHT_PRIOR_BUFFER=
+  _ZSH_HIGHLIGHT_PRIOR_CURSOR=
 }
-add-zsh-hook preexec _zsh_highlight_preexec_hook
+autoload -U add-zsh-hook
+add-zsh-hook preexec _zsh_highlight_preexec_hook 2>/dev/null || {
+    echo 'zsh-syntax-highlighting: failed loading add-zsh-hook.' >&2
+  }
 
 # Initialize the array of active highlighters if needed.
 [[ $#ZSH_HIGHLIGHT_HIGHLIGHTERS -eq 0 ]] && ZSH_HIGHLIGHT_HIGHLIGHTERS=(main) || true