diff --git a/04.md b/04.md
index 2e97a4a..d57a1b3 100644
--- a/04.md
+++ b/04.md
@@ -163,7 +163,7 @@ If the invoice was not paid yet, `Bob` responds with an error. In that case, `Al
 
 ## Cached Responses
 
-`Bob` **MUST** cache every successful `PostMintBolt11Response` using `PostMintBolt11Request` as key.
+`Bob` **MUST** cache every successful `PostMintBolt11Response` using `PostMintBolt11Request` as the key.
 Each cache entry has a TTL (Time-To-Live) of at least 1800 seconds.
 In the event of a network error, `Alice` **CAN** replay the same exact request and get the same response.