diff --git a/spec/Overview.html b/spec/Overview.html
index 5a8000b..d47e9e3 100644
--- a/spec/Overview.html
+++ b/spec/Overview.html
@@ -1257,7 +1257,7 @@ <h2>SubtleCrypto interface</h2>
                          sequence&lt;KeyUsage> keyUsages );
   Promise&lt;ArrayBuffer> deriveBits(AlgorithmIdentifier algorithm,
                           CryptoKey baseKey,
-                          unsigned long length);
+                          optional unsigned long? length = null);
 
   Promise&lt;CryptoKey> importKey(KeyFormat format,
                          (BufferSource or JsonWebKey) keyData,