diff --git a/book/source/13-armor.md b/book/source/13-armor.md
index 7928b46..5531f19 100644
--- a/book/source/13-armor.md
+++ b/book/source/13-armor.md
@@ -21,7 +21,7 @@ OpenPGP's ASCII armor mechanism consists of:
 
 ## Example
 
-In chapter {numref}`zoom_certificates`, we take a look at a very minimal variant of Alice's certificate. In ASCII armored form, the certificate is 388 bytes long, and looks like this:
+In the chapter {ref}`zoom_certificates`, we take a look at a very minimal variant of Alice's certificate. In ASCII armored form, the certificate is 388 bytes long, and looks like this:
 
 ```text
 $ cat alice_minimal.pub