diff --git a/slow/responses.py b/slow/responses.py
index d809e02..159a51c 100644
--- a/slow/responses.py
+++ b/slow/responses.py
@@ -17,7 +17,7 @@ class Response:
         self.body = body
         self.no_header = False
 
-    def header(self, app: App) -> bytes:
+    def header(self, app: "App") -> bytes:
         header = f"HTTP/1.1 {self.status} {http.client.responses.get(self.status, 'Unkown Status Code')}\r\n"
         header += f"Content-Length: {len(self.body)}\r\n"
         for h in self.headers: