diff --git a/tests/test-highlighting.zsh b/tests/test-highlighting.zsh
index f6ebff4..743b3bb 100755
--- a/tests/test-highlighting.zsh
+++ b/tests/test-highlighting.zsh
@@ -29,6 +29,8 @@
 # -------------------------------------------------------------------------------------------------
 
 
+setopt NOUNSET
+
 # Check an highlighter was given as argument.
 [[ -n "$1" ]] || {
   echo >&2 "Bail out! You must provide the name of a valid highlighter as argument."