diff --git a/packages/webview_flutter/webview_flutter_wkwebview/CHANGELOG.md b/packages/webview_flutter/webview_flutter_wkwebview/CHANGELOG.md
index b4fc793df82..a4ea405f961 100644
--- a/packages/webview_flutter/webview_flutter_wkwebview/CHANGELOG.md
+++ b/packages/webview_flutter/webview_flutter_wkwebview/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 3.18.5
+
+* Fixes crash when sending undefined message via JavaScript channel.
+
 ## 3.18.4
 
 * Fixes crash when native `WKFrameInfo.request` is nil.
diff --git a/packages/webview_flutter/webview_flutter_wkwebview/example/integration_test/webview_flutter_test.dart b/packages/webview_flutter/webview_flutter_wkwebview/example/integration_test/webview_flutter_test.dart
index 24e70d775b1..83a13e3d6bb 100644
--- a/packages/webview_flutter/webview_flutter_wkwebview/example/integration_test/webview_flutter_test.dart
+++ b/packages/webview_flutter/webview_flutter_wkwebview/example/integration_test/webview_flutter_test.dart
@@ -278,6 +278,47 @@ Future<void> main() async {
     await expectLater(channelCompleter.future, completion('hello'));
   });
 
+  testWidgets('JavaScriptChannel can receive undefined',
+      (WidgetTester tester) async {
+    final Completer<void> pageFinished = Completer<void>();
+    final PlatformWebViewController controller = PlatformWebViewController(
+      const PlatformWebViewControllerCreationParams(),
+    );
+    unawaited(controller.setJavaScriptMode(JavaScriptMode.unrestricted));
+    final PlatformNavigationDelegate delegate = PlatformNavigationDelegate(
+      const PlatformNavigationDelegateCreationParams(),
+    );
+    unawaited(delegate.setOnPageFinished((_) => pageFinished.complete()));
+    unawaited(controller.setPlatformNavigationDelegate(delegate));
+
+    final Completer<String> channelCompleter = Completer<String>();
+    await controller.addJavaScriptChannel(
+      JavaScriptChannelParams(
+        name: 'Channel',
+        onMessageReceived: (JavaScriptMessage message) {
+          channelCompleter.complete(message.message);
+        },
+      ),
+    );
+
+    await controller.loadHtmlString(
+      'data:text/html;charset=utf-8;base64,PCFET0NUWVBFIGh0bWw+',
+    );
+
+    await tester.pumpWidget(Builder(
+      builder: (BuildContext context) {
+        return PlatformWebViewWidget(
+          PlatformWebViewWidgetCreationParams(controller: controller),
+        ).build(context);
+      },
+    ));
+
+    await pageFinished.future;
+
+    await controller.runJavaScript('Channel.postMessage(undefined);');
+    await expectLater(channelCompleter.future, completion('(null)'));
+  });
+
   testWidgets('resize webview', (WidgetTester tester) async {
     final Completer<void> buttonTapResizeCompleter = Completer<void>();
     final Completer<void> onPageFinished = Completer<void>();
diff --git a/packages/webview_flutter/webview_flutter_wkwebview/lib/src/common/weak_reference_utils.dart b/packages/webview_flutter/webview_flutter_wkwebview/lib/src/common/weak_reference_utils.dart
index f12485ad6e7..014d9eb42ea 100644
--- a/packages/webview_flutter/webview_flutter_wkwebview/lib/src/common/weak_reference_utils.dart
+++ b/packages/webview_flutter/webview_flutter_wkwebview/lib/src/common/weak_reference_utils.dart
@@ -18,7 +18,7 @@
 ///       ) {
 ///         weakReference.target?.onJavascriptChannelMessage(
 ///           message.name,
-///           message.body!.toString(),
+///           message.body.toString(),
 ///         );
 ///       };
 ///     },
diff --git a/packages/webview_flutter/webview_flutter_wkwebview/lib/src/legacy/web_kit_webview_widget.dart b/packages/webview_flutter/webview_flutter_wkwebview/lib/src/legacy/web_kit_webview_widget.dart
index 016f1734b94..89e184a89a5 100644
--- a/packages/webview_flutter/webview_flutter_wkwebview/lib/src/legacy/web_kit_webview_widget.dart
+++ b/packages/webview_flutter/webview_flutter_wkwebview/lib/src/legacy/web_kit_webview_widget.dart
@@ -456,7 +456,7 @@ class WebKitWebViewPlatformController extends WebViewPlatformController {
                 ) {
                   weakReference.target?.onJavascriptChannelMessage(
                     message.name,
-                    message.body!.toString(),
+                    message.body.toString(),
                   );
                 };
               },
diff --git a/packages/webview_flutter/webview_flutter_wkwebview/lib/src/webkit_webview_controller.dart b/packages/webview_flutter/webview_flutter_wkwebview/lib/src/webkit_webview_controller.dart
index 2ece1206dae..843c73f3de3 100644
--- a/packages/webview_flutter/webview_flutter_wkwebview/lib/src/webkit_webview_controller.dart
+++ b/packages/webview_flutter/webview_flutter_wkwebview/lib/src/webkit_webview_controller.dart
@@ -905,8 +905,13 @@ class WebKitJavaScriptChannelParams extends JavaScriptChannelParams {
             (WeakReference<void Function(JavaScriptMessage)> weakReference) {
               return (_, __, WKScriptMessage message) {
                 if (weakReference.target != null) {
+                  // When message.body is null, return '(null)' for consistency
+                  // with previous implementations.
                   weakReference.target!(
-                    JavaScriptMessage(message: message.body!.toString()),
+                    JavaScriptMessage(
+                        message: message.body == null
+                            ? '(null)'
+                            : message.body.toString()),
                   );
                 }
               };
diff --git a/packages/webview_flutter/webview_flutter_wkwebview/pubspec.yaml b/packages/webview_flutter/webview_flutter_wkwebview/pubspec.yaml
index 5a0e7ed115a..67f2f644cd6 100644
--- a/packages/webview_flutter/webview_flutter_wkwebview/pubspec.yaml
+++ b/packages/webview_flutter/webview_flutter_wkwebview/pubspec.yaml
@@ -2,7 +2,7 @@ name: webview_flutter_wkwebview
 description: A Flutter plugin that provides a WebView widget based on Apple's WKWebView control.
 repository: https://github.com/flutter/packages/tree/main/packages/webview_flutter/webview_flutter_wkwebview
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+webview%22
-version: 3.18.4
+version: 3.18.5
 
 environment:
   sdk: ^3.5.0