diff --git a/websocket-sharp/Net/WebHeaderCollection.cs b/websocket-sharp/Net/WebHeaderCollection.cs
index b8d044e2..e3cd5fa4 100644
--- a/websocket-sharp/Net/WebHeaderCollection.cs
+++ b/websocket-sharp/Net/WebHeaderCollection.cs
@@ -998,10 +998,14 @@ namespace WebSocketSharp.Net
     internal void InternalSet (string name, string value, bool response)
     {
       value = checkValue (value);
-      if (IsMultiValue (name, response))
+
+      if (IsMultiValue (name, response)) {
         base.Add (name, value);
-      else
-        base.Set (name, value);
+
+        return;
+      }
+
+      base.Set (name, value);
     }
 
     internal static bool IsMultiValue (string headerName, bool response)