diff --git a/websocket-sharp/Net/WebHeaderCollection.cs b/websocket-sharp/Net/WebHeaderCollection.cs
index 1fb60dc7..9a12ab97 100644
--- a/websocket-sharp/Net/WebHeaderCollection.cs
+++ b/websocket-sharp/Net/WebHeaderCollection.cs
@@ -1021,6 +1021,12 @@ namespace WebSocketSharp.Net
     ///   -or-
     ///   </para>
     ///   <para>
+    ///   <paramref name="headerName"/> is a string of spaces.
+    ///   </para>
+    ///   <para>
+    ///   -or-
+    ///   </para>
+    ///   <para>
     ///   <paramref name="headerName"/> contains an invalid character.
     ///   </para>
     ///   <para>