diff --git a/index.bs b/index.bs
index 9e42d718..4c95f591 100644
--- a/index.bs
+++ b/index.bs
@@ -1004,6 +1004,8 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
 
       The <dfn method for="NavigationPreloadManager"><code>setHeaderValue(|value|)</code></dfn> method steps are:
 
+        1. Let |value| be the result of [=header value/normalizing=] |value|.
+        1. If |value| is not a [=header value=], return [=a promise rejected with=] a {{TypeError}}.
         1. Let |promise| be [=a new promise=].
         1. Run the following steps [=in parallel=]:
             1. Let |registration| be [=this=]'s associated [=/service worker registration=].