diff --git a/ox-hugo.el b/ox-hugo.el
index 80ef84f..6bd9421 100644
--- a/ox-hugo.el
+++ b/ox-hugo.el
@@ -3068,6 +3068,7 @@ are \"toml\" and \"yaml\"."
                      "HUGO_OUTPUTS"
                      "HUGO_TAGS"
                      "HUGO_CATEGORIES"
+                     "HUGO_SERIES"
                      "HUGO_TYPE"
                      "HUGO_LAYOUT"
                      "HUGO_WEIGHT"