diff --git a/websocket-sharp/WebSocket.cs b/websocket-sharp/WebSocket.cs
index d342d475..abc7e3a4 100644
--- a/websocket-sharp/WebSocket.cs
+++ b/websocket-sharp/WebSocket.cs
@@ -721,6 +721,13 @@ namespace WebSocketSharp
         return false;
       }
 
+      if (!_ignoreExtensions
+          && !validateSecWebSocketExtensionsClientHeader (headers["Sec-WebSocket-Extensions"])
+      ) {
+        message = "Includes an invalid Sec-WebSocket-Extensions header.";
+        return false;
+      }
+
       return true;
     }
 
@@ -1697,6 +1704,12 @@ namespace WebSocketSharp
       return value != null && value == CreateResponseKey (_base64Key);
     }
 
+    // As server
+    private bool validateSecWebSocketExtensionsClientHeader (string value)
+    {
+      return value == null || value.Length > 0;
+    }
+
     // As client
     private bool validateSecWebSocketExtensionsHeader (string value)
     {