diff --git a/aiogram/dispatcher/webhook.py b/aiogram/dispatcher/webhook.py
index e8246b59..11e3238c 100644
--- a/aiogram/dispatcher/webhook.py
+++ b/aiogram/dispatcher/webhook.py
@@ -145,7 +145,7 @@ class WebhookRequestHandler(web.View):
             web_response = web.Response(text='ok')
 
         if self.request.app.get('RETRY_AFTER', None):
-            web_response.headers['Retry-After'] = self.request.app['RETRY_AFTER']
+            web_response.headers['Retry-After'] = str(self.request.app['RETRY_AFTER'])
 
         return web_response