diff --git a/build-themes.sh b/build-themes.sh
index 3ed054d..3d36ed7 100755
--- a/build-themes.sh
+++ b/build-themes.sh
@@ -9,8 +9,8 @@ if [[ "$1" == "--watch" && -n "$2" ]]; then
   [[ $(basename "$file") = _* ]] && exit 1
   sass --no-source-map --watch "$file" "$css_dir/theme-$(basename "$file" .scss).css"
 else
+  set -u
   rm -r "$css_dir/"
-
   #build default first
   sass --no-source-map "$sass_dir/_default.scss" "$css_dir/style.css"
   for file in "$sass_dir"/*.scss; do