diff --git a/docs/api.md b/docs/api.md
index 9bcfe31f..bcb114e8 100644
--- a/docs/api.md
+++ b/docs/api.md
@@ -2189,6 +2189,8 @@ This setting will change the default maximum time for the following methods and
 
 The extra HTTP headers will be sent with every request the page initiates.
 
+> **NOTE** All HTTP header names are lowercased. (HTTP headers are case-insensitive, so this shouldn’t impact your server code.)
+
 > **NOTE** page.setExtraHTTPHeaders does not guarantee the order of headers in the outgoing requests.
 
 #### page.setGeolocation(options)