diff --git a/Makefile b/Makefile
index f454324..2150946 100644
--- a/Makefile
+++ b/Makefile
@@ -39,7 +39,7 @@ test:
 	for test in highlighters/*; do \
 		if [ -d $$test/test-data ]; then \
 			echo "Running test $${test##*/}"; \
-			$(ZSH) -f tests/test-highlighting.zsh "$${test##*/}"; \
+			$(ZSH) -fu tests/test-highlighting.zsh "$${test##*/}"; \
 			: $$(( result |= $$? )); \
 		fi \
 	done; \
diff --git a/highlighters/brackets/brackets-highlighter.zsh b/highlighters/brackets/brackets-highlighter.zsh
index 1bdd1f9..335f7a8 100644
--- a/highlighters/brackets/brackets-highlighter.zsh
+++ b/highlighters/brackets/brackets-highlighter.zsh
@@ -40,7 +40,7 @@
 # Whether the brackets highlighter should be called or not.
 _zsh_highlight_highlighter_brackets_predicate()
 {
-  [[ $WIDGET == zle-line-finish ]] || _zsh_highlight_cursor_moved || _zsh_highlight_buffer_modified
+  [[ ${WIDGET-} == zle-line-finish ]] || _zsh_highlight_cursor_moved || _zsh_highlight_buffer_modified
 }
 
 # Brackets highlighting function.
@@ -59,7 +59,7 @@ _zsh_highlight_highlighter_brackets_paint()
         lastoflevel[$level]=$pos
         ;;
       [")]}"])
-        matchingpos=$lastoflevel[$level]
+        matchingpos=${lastoflevel[$level]-}
         levelpos[$pos]=$((level--))
 	if _zsh_highlight_brackets_match $matchingpos $pos; then
           matching[$matchingpos]=$pos
@@ -86,9 +86,9 @@ _zsh_highlight_highlighter_brackets_paint()
   done
 
   # If cursor is on a bracket, then highlight corresponding bracket, if any.
-  if [[ $WIDGET != zle-line-finish ]]; then
+  if [[ ${WIDGET-} != zle-line-finish ]]; then
     pos=$((CURSOR + 1))
-    if [[ -n $levelpos[$pos] ]] && [[ -n $matching[$pos] ]]; then
+    if [[ -n ${levelpos[$pos]-} && -n $matching[$pos] ]]; then
       local -i otherpos=$matching[$pos]
       _zsh_highlight_add_highlight $((otherpos - 1)) $otherpos cursor-matchingbracket
     fi
@@ -98,7 +98,7 @@ _zsh_highlight_highlighter_brackets_paint()
 # Helper function to differentiate type 
 _zsh_highlight_brackets_match()
 {
-  case $BUFFER[$1] in
+  case ${BUFFER[$1]-} in
     \() [[ $BUFFER[$2] == \) ]];;
     \[) [[ $BUFFER[$2] == \] ]];;
     \{) [[ $BUFFER[$2] == \} ]];;
diff --git a/highlighters/main/main-highlighter.zsh b/highlighters/main/main-highlighter.zsh
index 1bbc3cb..276964c 100644
--- a/highlighters/main/main-highlighter.zsh
+++ b/highlighters/main/main-highlighter.zsh
@@ -58,7 +58,7 @@
 _zsh_highlight_highlighter_main_predicate()
 {
   # may need to remove path_prefix highlighting when the line ends
-  [[ $WIDGET == zle-line-finish ]] || _zsh_highlight_buffer_modified
+  [[ ${WIDGET-} == zle-line-finish ]] || _zsh_highlight_buffer_modified
 }
 
 # Helper to deal with tokens crossing line boundaries.
diff --git a/tests/test-highlighting.zsh b/tests/test-highlighting.zsh
index 243f846..9cedef4 100755
--- a/tests/test-highlighting.zsh
+++ b/tests/test-highlighting.zsh
@@ -56,7 +56,7 @@ zmodload zsh/zle
 # Overwrite _zsh_highlight_add_highlight so we get the key itself instead of the style
 _zsh_highlight_add_highlight()
 {
-  region_highlight+=("$1 $2 $3")
+  region_highlight+=("$1 $2 ${3-}")
 }
 
 # Activate the highlighter.
@@ -96,7 +96,7 @@ run_test_internal() {
       (( --end )) # convert to closed range, like expected_region_highlight
       (( ++start, ++end )) # region_highlight is 0-indexed; expected_region_highlight is 1-indexed
       for j in {$start..$end}; do
-        observed_result[$j]=$highlight_zone[3]
+        observed_result[$j]=${highlight_zone[3]-}
       done
     else
       # noop range; ignore.
@@ -115,10 +115,10 @@ run_test_internal() {
     local desc="[$start,$end] «${BUFFER[$start,$end]//'#'/♯}»"
     # Match the emptiness of observed_result if no highlighting is expected
     [[ $highlight_zone[3] == NONE ]] && highlight_zone[3]=
-    [[ -n "$highlight_zone[4]" ]] && todo="# TODO $highlight_zone[4]"
+    [[ -n ${highlight_zone[4]-} ]] && todo="# TODO $highlight_zone[4]"
     for j in {$start..$end}; do
-      if [[ "$observed_result[$j]" != "$highlight_zone[3]" ]]; then
-        print -r -- "not ok $i - $desc - expected ${(qqq)highlight_zone[3]}, observed ${(qqq)observed_result[$j]}. $todo"
+      if [[ ${observed_result[$j]-} != $highlight_zone[3] ]]; then
+        print -r -- "not ok $i - $desc - expected ${(qqq)highlight_zone[3]}, observed ${(qqq)observed_result[$j]-}. $todo"
         continue 2
       fi
     done
@@ -162,7 +162,7 @@ run_test() {
 
 # Set up results_filter
 local results_filter
-if [[ $QUIET == y ]]; then
+if [[ ${QUIET-} == y ]]; then
   if type -w perl >/dev/null; then
     results_filter=${0:A:h}/tap-filter
   else
diff --git a/zsh-syntax-highlighting.zsh b/zsh-syntax-highlighting.zsh
index 7e1cdc6..5db4b8a 100644
--- a/zsh-syntax-highlighting.zsh
+++ b/zsh-syntax-highlighting.zsh
@@ -104,7 +104,7 @@ _zsh_highlight()
 
   # Remove all highlighting in isearch, so that only the underlining done by zsh itself remains.
   # For details see FAQ entry 'Why does syntax highlighting not work while searching history?'.
-  if [[ $WIDGET == zle-isearch-update ]] && ! (( $+ISEARCHMATCH_ACTIVE )); then
+  if [[ ${WIDGET-} == zle-isearch-update ]] && ! (( $+ISEARCHMATCH_ACTIVE )); then
     region_highlight=()
     return $ret
   fi
@@ -118,7 +118,7 @@ _zsh_highlight()
   [[ -n ${ZSH_HIGHLIGHT_MAXLENGTH:-} ]] && [[ $#BUFFER -gt $ZSH_HIGHLIGHT_MAXLENGTH ]] && return $ret
 
   # Do not highlight if there are pending inputs (copy/paste).
-  [[ $PENDING -gt 0 ]] && return $ret
+  [[ ${PENDING-} -gt 0 ]] && return $ret
 
   # Reset region highlight to build it from scratch
   typeset -ga region_highlight
@@ -198,7 +198,7 @@ _zsh_highlight()
 
   } always {
     typeset -g _ZSH_HIGHLIGHT_PRIOR_BUFFER="$BUFFER"
-    typeset -gi _ZSH_HIGHLIGHT_PRIOR_CURSOR=$CURSOR
+    typeset -gi _ZSH_HIGHLIGHT_PRIOR_CURSOR=${CURSOR-}
   }
 }
 
@@ -213,10 +213,8 @@ _zsh_highlight_apply_zle_highlight() {
   local entry="$1" default="$2"
   integer first="$3" second="$4"
 
-  setopt localoptions unset	# Is it bug or feature that nounset will often abort this function?
-
   # read the relevant entry from zle_highlight
-  local region="${zle_highlight[(r)${entry}:*]}"
+  local region="${zle_highlight[(r)${entry}:*]-}"
 
   if [[ -z "$region" ]]; then
     # entry not specified at all, use default value
@@ -261,7 +259,7 @@ _zsh_highlight_buffer_modified()
 # Returns 0 if the cursor has moved since _zsh_highlight was last called.
 _zsh_highlight_cursor_moved()
 {
-  [[ -n $CURSOR ]] && [[ -n ${_ZSH_HIGHLIGHT_PRIOR_CURSOR-} ]] && (($_ZSH_HIGHLIGHT_PRIOR_CURSOR != $CURSOR))
+  [[ -n ${CURSOR-} && -n ${_ZSH_HIGHLIGHT_PRIOR_CURSOR-} ]] && (($_ZSH_HIGHLIGHT_PRIOR_CURSOR != $CURSOR))
 }
 
 # Add a highlight defined by ZSH_HIGHLIGHT_STYLES.
@@ -309,8 +307,17 @@ then
     } "$@"
   }
   _zsh_highlight_bind_widgets(){}
+
+  local prevunsetstate=
+  [[ -o unset ]] || prevunsetstate=NO_
+  setopt localoptions UNSET	# for following two add-zle-hook-widget calls...
+  # TODO: figure out why only 5.3 with no_unset gives following error
+  # add-zle-hook-widget:84: widgets[$hook]: parameter not set
+
   add-zle-hook-widget zle-line-pre-redraw _zsh_highlight
   add-zle-hook-widget zle-line-finish _zsh_highlight__zle-line-finish
+
+  setopt noxtrace ${prevunsetstate}UNSET	# put back as before
 else
   # Rebind all ZLE widgets to make them invoke _zsh_highlights.
   _zsh_highlight_bind_widgets()
@@ -340,7 +347,7 @@ else
   
     local cur_widget
     for cur_widget in $widgets_to_bind; do
-      case $widgets[$cur_widget] in
+      case ${widgets[$cur_widget]-} in
   
         # Already rebound event: do nothing.
         user:_zsh_highlight_widget_*);;
@@ -367,7 +374,7 @@ else
   
         # Incomplete or nonexistent widget: Bind to z-sy-h directly.
         *) 
-           if [[ $cur_widget == zle-* ]] && [[ -z $widgets[$cur_widget] ]]; then
+           if [[ $cur_widget == zle-* && -z ${widgets[$cur_widget]-} ]]; then
              _zsh_highlight_widget_${cur_widget}() { :; _zsh_highlight }
              zle -N $cur_widget _zsh_highlight_widget_$cur_widget
            else