diff --git a/pkgs/cupertino_http/lib/src/cupertino_web_socket.dart b/pkgs/cupertino_http/lib/src/cupertino_web_socket.dart
index b374dd9907..aa1ecd3aa1 100644
--- a/pkgs/cupertino_http/lib/src/cupertino_web_socket.dart
+++ b/pkgs/cupertino_http/lib/src/cupertino_web_socket.dart
@@ -6,6 +6,7 @@ import 'dart:async';
 import 'dart:convert';
 import 'dart:typed_data';
 
+import 'package:cupertino_http/src/utils.dart';
 import 'package:web_socket/web_socket.dart';
 
 import 'cupertino_api.dart';
@@ -206,11 +207,10 @@ class CupertinoWebSocket implements WebSocket {
     if (_events.isClosed) {
       throw WebSocketConnectionClosed();
     }
+  
+    checkCloseCodeRfc(code);
+    checkCloseReason(reason);
 
-    if (code != null && code != 1000 && !(code >= 3000 && code <= 4999)) {
-      throw ArgumentError('Invalid argument: $code, close code must be 1000 or '
-          'in the range 3000-4999');
-    }
     if (reason != null && utf8.encode(reason).length > 123) {
       throw ArgumentError.value(reason, 'reason',
           'reason must be <= 123 bytes long when encoded as UTF-8');
diff --git a/pkgs/cupertino_http/lib/src/utils.dart b/pkgs/cupertino_http/lib/src/utils.dart
index 7bc87aa87c..aad01d4fd1 100644
--- a/pkgs/cupertino_http/lib/src/utils.dart
+++ b/pkgs/cupertino_http/lib/src/utils.dart
@@ -2,6 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'dart:convert';
 import 'dart:ffi';
 import 'dart:io';
 
@@ -90,3 +91,27 @@ ncb.NSArray stringIterableToNSArray(Iterable<String> strings) {
 
 ncb.NSURL uriToNSURL(Uri uri) => ncb.NSURL
     .URLWithString_(linkedLibs, uri.toString().toNSString(linkedLibs))!;
+
+
+/// Throw if the given close code is not valid according to RFC 6455.
+/// See https://www.rfc-editor.org/rfc/rfc6455.html#section-7.4
+void checkCloseCodeRfc(int? code) {
+  const reservedCloseCodes = [1004, 1005, 1006];
+  if (code != null &&
+      !(code >= 1000 && code <= 1011 && !reservedCloseCodes.contains(code)) &&
+      !(code >= 3000 && code <= 4999)) {
+    throw ArgumentError(
+      'Invalid argument: $code, close code must be in the range 1000-1011 or '
+      'in the range 3000-4999, and cannot be one of reserved codes '
+      '(${reservedCloseCodes.join(', ')})',
+    );
+  }
+}
+
+/// Throw if the given close reason is not valid.
+void checkCloseReason(String? reason) {
+  if (reason != null && utf8.encode(reason).length > 123) {
+    throw ArgumentError.value(reason, 'reason',
+        'reason must be <= 123 bytes long when encoded as UTF-8');
+  }
+}
diff --git a/pkgs/web_socket/lib/src/browser_web_socket.dart b/pkgs/web_socket/lib/src/browser_web_socket.dart
index fc66628b7a..f18b332b56 100644
--- a/pkgs/web_socket/lib/src/browser_web_socket.dart
+++ b/pkgs/web_socket/lib/src/browser_web_socket.dart
@@ -125,7 +125,7 @@ class BrowserWebSocket implements WebSocket {
       throw WebSocketConnectionClosed();
     }
 
-    checkCloseCode(code);
+    checkCloseCodeWeb(code);
     checkCloseReason(reason);
 
     unawaited(_events.close());
diff --git a/pkgs/web_socket/lib/src/fake_web_socket.dart b/pkgs/web_socket/lib/src/fake_web_socket.dart
index 1e4d07198a..c74e743502 100644
--- a/pkgs/web_socket/lib/src/fake_web_socket.dart
+++ b/pkgs/web_socket/lib/src/fake_web_socket.dart
@@ -23,7 +23,7 @@ class FakeWebSocket implements WebSocket {
       throw WebSocketConnectionClosed();
     }
 
-    checkCloseCode(code);
+    checkCloseCodeWeb(code);
     checkCloseReason(reason);
 
     unawaited(_events.close());
diff --git a/pkgs/web_socket/lib/src/io_web_socket.dart b/pkgs/web_socket/lib/src/io_web_socket.dart
index 7ea084bec3..f6bfe4a952 100644
--- a/pkgs/web_socket/lib/src/io_web_socket.dart
+++ b/pkgs/web_socket/lib/src/io_web_socket.dart
@@ -105,7 +105,7 @@ class IOWebSocket implements WebSocket {
       throw WebSocketConnectionClosed();
     }
 
-    checkCloseCode(code);
+    checkCloseCodeRfc(code);
     checkCloseReason(reason);
 
     unawaited(_events.close());
diff --git a/pkgs/web_socket/lib/src/utils.dart b/pkgs/web_socket/lib/src/utils.dart
index 7331840f62..19f8939d22 100644
--- a/pkgs/web_socket/lib/src/utils.dart
+++ b/pkgs/web_socket/lib/src/utils.dart
@@ -4,14 +4,37 @@
 
 import 'dart:convert';
 
-/// Throw if the given close code is not valid.
-void checkCloseCode(int? code) {
+/// Throw if the given close code is not valid according to WHATWG spec.
+/// 
+/// This is more suitable for clients running in web browsers.
+/// 
+/// See https://websockets.spec.whatwg.org/#dom-websocket-close.
+void checkCloseCodeWeb(int? code) {
   if (code != null && code != 1000 && !(code >= 3000 && code <= 4999)) {
     throw ArgumentError('Invalid argument: $code, close code must be 1000 or '
         'in the range 3000-4999');
   }
 }
 
+/// Throw if the given close code is not valid according to RFC 6455.
+/// 
+/// This is more suitable for clients running in native environments, possibly
+/// as a server endpoint.
+/// 
+/// See https://www.rfc-editor.org/rfc/rfc6455.html#section-7.4
+void checkCloseCodeRfc(int? code) {
+  const reservedCloseCodes = [1004, 1005, 1006];
+  if (code != null &&
+      !(code >= 1000 && code <= 1011 && !reservedCloseCodes.contains(code)) &&
+      !(code >= 3000 && code <= 4999)) {
+    throw ArgumentError(
+      'Invalid argument: $code, close code must be in the range 1000-1011 or '
+      'in the range 3000-4999, and cannot be one of reserved codes '
+      '(${reservedCloseCodes.join(', ')})',
+    );
+  }
+}
+
 /// Throw if the given close reason is not valid.
 void checkCloseReason(String? reason) {
   if (reason != null && utf8.encode(reason).length > 123) {
diff --git a/pkgs/web_socket/lib/src/web_socket.dart b/pkgs/web_socket/lib/src/web_socket.dart
index 255c6e6c6d..d938ad1561 100644
--- a/pkgs/web_socket/lib/src/web_socket.dart
+++ b/pkgs/web_socket/lib/src/web_socket.dart
@@ -150,7 +150,10 @@ abstract interface class WebSocket {
   /// [code] is set then the peer will see a 1005 status code. If no [reason]
   /// is set then the peer will not receive a reason string.
   ///
-  /// Throws an [ArgumentError] if [code] is not 1000 or in the range 3000-4999.
+  /// Throws an [ArgumentError] if [code] is not accepted by the implementation.
+  /// All implementations must accept [code] values of 1000, and in the range
+  /// 3000-4999. Some implementations may accept additional values, but must not
+  /// accept reserved codes 1004, 1005, 1006, and 1015.
   ///
   /// Throws an [ArgumentError] if [reason] is longer than 123 bytes when
   /// encoded as UTF-8
diff --git a/pkgs/web_socket_conformance_tests/lib/src/close_local_tests.dart b/pkgs/web_socket_conformance_tests/lib/src/close_local_tests.dart
index cf39f95eb3..f069da87b1 100644
--- a/pkgs/web_socket_conformance_tests/lib/src/close_local_tests.dart
+++ b/pkgs/web_socket_conformance_tests/lib/src/close_local_tests.dart
@@ -55,11 +55,16 @@ void testCloseLocal(
       httpServerChannel.sink.add(null);
     });
 
-    test('reserved close code: 1004', () async {
-      final channel = await channelFactory(uri);
-      await expectLater(
-          () => channel.close(1004), throwsA(isA<ArgumentError>()));
-    });
+    /// Codes 1004-1006 and 1015 cannot be used in close() calls.
+    for (final reservedCode in [1004, 1005, 1006, 1015]) {
+      test('reserved close code: $reservedCode', () async {
+        final channel = await channelFactory(uri);
+        await expectLater(
+          () => channel.close(reservedCode),
+          throwsA(isA<ArgumentError>()),
+        );
+      });
+    }
 
     test('reserved close code: 2999', () async {
       final channel = await channelFactory(uri);