diff --git a/src/line-bot-sdk-csharp/Webhooks/WebhookEventSource.cs b/src/line-bot-sdk-csharp/Webhooks/WebhookEventSource.cs
index e9283e8..2dc07d5 100644
--- a/src/line-bot-sdk-csharp/Webhooks/WebhookEventSource.cs
+++ b/src/line-bot-sdk-csharp/Webhooks/WebhookEventSource.cs
@@ -33,7 +33,7 @@ internal static WebhookEventSource CreateFrom(dynamic source)
             {
                 return null;
             }
-            var sourceId = "";
+            string sourceId;
             switch (sourceType)
             {
                 case EventSourceType.User: