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pkgs/cupertino_http/lib/src/websocket.dart

@@ -96,7 +96,7 @@ class CupertinoWebSocket implements XXXWebSocket {
   void _closed(int? closeCode, Data? reason) {
     print('closing with $closeCode');
     if (!_events.isClosed) {
-      final closeReason = reason == null ? null : utf8.decode(reason.bytes);
+      final closeReason = reason == null ? '' : utf8.decode(reason.bytes);
 
       _events
         ..add(CloseReceived(closeCode, closeReason))
@@ -126,6 +126,18 @@ class CupertinoWebSocket implements XXXWebSocket {
 
   @override
   Future<void> close([int? code, String? reason]) async {
+    if (_events.isClosed) {
+      throw XXXWebSocketConnectionClosed();
+    }
+
+    if (code != null) {
+      RangeError.checkValueInInterval(code, 3000, 4999, 'code');
+    }
+    if (reason != null && utf8.encode(reason).length > 123) {
+      throw ArgumentError.value(reason, 'reason',
+          'reason must be <= 123 bytes long when encoded as UTF-8');
+    }
+
     if (!_events.isClosed) {
       unawaited(_events.close());
 

pkgs/websocket/lib/iowebsocket.dart

@@ -40,8 +40,8 @@ class IOWebSocket implements XXXWebSocket {
       onDone: () {
         print('onDone');
         if (!_events.isClosed) {
-          _events
-              .add(CloseReceived(_webSocket.closeCode, _webSocket.closeReason));
+          _events.add(CloseReceived(
+              _webSocket.closeCode, _webSocket.closeReason ?? ""));
           _events.close();
         }
       },
@@ -80,15 +80,15 @@ class IOWebSocket implements XXXWebSocket {
   //  endpoint that has already sent a Close frame will continue to process
   //  data.
   @override
-  Future<void> close([int? code, String reason = '']) async {
+  Future<void> close([int? code, String? reason]) async {
     if (_events.isClosed) {
       throw XXXWebSocketConnectionClosed();
     }
 
     if (code != null) {
       RangeError.checkValueInInterval(code, 3000, 4999, 'code');
     }
-    if (utf8.encode(reason).length > 123) {
+    if (reason != null && utf8.encode(reason).length > 123) {
       throw ArgumentError.value(reason, "reason",
           "reason must be <= 123 bytes long when encoded as UTF-8");
     }

pkgs/websocket/lib/websocket.dart

@@ -108,7 +108,7 @@ abstract interface class XXXWebSocket {
   /// Sends a Close frame to the peer. If the optional [code] and [reason]
   /// arguments are given, they will be included in the Close frame. If no
   /// [code] is set then the peer will see a 1005 status code. If no [reason]
-  /// is set then the peer will receive an empty reason string.
+  /// is set then the peer will not receive a reason string.
   ///
   /// Throws a [RangeError] if [code] is not in the range 3000-4999.
   ///
@@ -117,7 +117,7 @@ abstract interface class XXXWebSocket {
   ///
   /// Throws [XXXWebSocketConnectionClosed] if the connection is already closed
   /// (including by the peer).
-  Future<void> close([int? code, String reason = '']);
+  Future<void> close([int? code, String? reason]);
 
   /// A [Stream] of [WebSocketEvent] received from the peer.
   ///