diff --git a/src/epub/content.opf b/src/epub/content.opf
index 3bed5db..5b28dff 100644
--- a/src/epub/content.opf
+++ b/src/epub/content.opf
@@ -22,6 +22,7 @@
alternativeText
longDescription
readingOrder
+ structuralNavigation
tableOfContents
unlocked
none