diff --git a/book.toml b/book.toml
index 94f5a92..7a8c445 100644
--- a/book.toml
+++ b/book.toml
@@ -10,7 +10,9 @@ git-repository-url = "https://github.com/hust-open-atom-club/linux-insides-zh"
 site-url = "/linux-insides-zh/"
 
 [output.pdf]
+optional = true
 cover-image = "cover.jpg"
 
 [output.epub]
+optional = true
 cover-image = "cover.jpg"