diff --git a/websocket-sharp/Ext.cs b/websocket-sharp/Ext.cs
index a66637e4..310e1e9b 100644
--- a/websocket-sharp/Ext.cs
+++ b/websocket-sharp/Ext.cs
@@ -1356,6 +1356,9 @@ namespace WebSocketSharp
     /// <param name="address">
     /// A <see cref="System.Net.IPAddress"/> to test.
     /// </param>
+    /// <exception cref="ArgumentNullException">
+    /// <paramref name="address"/> is <see langword="null"/>.
+    /// </exception>
     public static bool IsLocal (this System.Net.IPAddress address)
     {
       if (address == null)