diff --git a/api/webhook_manager.go b/api/webhook_manager.go
index d36ba43..3a43eb6 100644
--- a/api/webhook_manager.go
+++ b/api/webhook_manager.go
@@ -67,7 +67,7 @@ func (whm *WebhookManager) sendWebhooks(request interface{}, r *http.Request) {
 			payload := toNormalizedJson(payloadBytes)
 
 			signature := generatePayloadSignature(payload, app.WebhookSecret)
-			req.Header.Add("X-Hub-Signature-256", signature)
+			req.Header.Set("X-Hub-Signature-256", signature)
 
 			client := &http.Client{}
 			resp, err := client.Do(req)