diff --git a/src/epub/content.opf b/src/epub/content.opf
index e77c52b..9e9f704 100644
--- a/src/epub/content.opf
+++ b/src/epub/content.opf
@@ -22,6 +22,7 @@
alternativeText
longDescription
readingOrder
+ structuralNavigation
tableOfContents
unlocked
none