diff --git a/sig/server.rbs b/sig/server.rbs
index 2565680..74db11f 100644
--- a/sig/server.rbs
+++ b/sig/server.rbs
@@ -30,7 +30,7 @@ module WEBrick
 
     def listen: (String address, Integer port) -> void
 
-    def start: () { (TCPSocket) -> void } -> void
+    def start: () ?{ (TCPSocket) -> void } -> void
 
     def stop: () -> void
 
@@ -42,7 +42,7 @@ module WEBrick
 
     def accept_client: (TCPServer svr) -> TCPSocket?
 
-    def start_thread: (TCPSocket sock) { (TCPSocket) -> void } -> Thread
+    def start_thread: (TCPSocket sock) ?{ (TCPSocket) -> void } -> Thread
 
     def call_callback: (Symbol callback_name, *untyped args) -> untyped