@@ -11,6 +11,7 @@ library;
 import 'package:flutter/foundation.dart';
 
 import 'basic_types.dart';
+import 'debug.dart';
 
 /// Base class for [BorderRadius] that allows for text-direction aware resolution.
 ///
@@ -785,7 +786,7 @@ class BorderRadiusDirectional extends BorderRadiusGeometry {
 
   @override
   BorderRadius resolve(TextDirection? direction) {
-    assert(direction != null);
+    assert(debugCheckCanResolveTextDirection(direction, '$BorderRadiusDirectional'));
     switch (direction!) {
       case TextDirection.rtl:
         return BorderRadius.only(
 
@@ -16,7 +16,7 @@
 library;
 
 import 'dart:io';
-import 'dart:ui' show Image, Picture, Size;
+import 'dart:ui' show Image, Picture, Size, TextDirection;
 
 import 'package:flutter/foundation.dart';
 
@@ -232,3 +232,40 @@ bool _defaultPictureCapture(Picture picture) => true;
 /// Tests may use this to capture the picture and run assertions on it.
 ShaderWarmUpImageCallback debugCaptureShaderWarmUpImage = _defaultImageCapture;
 bool _defaultImageCapture(Image image) => true;
+
+/// Asserts that a given [TextDirection] is not null.
+///
+/// Used by painting library classes that require a [TextDirection] to resolve
+/// their properties, such as [BorderRadiusDirectional].
+///
+/// Does nothing if asserts are disabled. Always returns true.
+///
+/// See also:
+///
+///  * [debugCheckHasDirectionality], which is a similar widgets-library level function.
+bool debugCheckCanResolveTextDirection(TextDirection? direction, String target) {
+  assert(() {
+    if (direction == null) {
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('No TextDirection found.'),
+        ErrorDescription(
+          'To resolve $target properties, it must be provided with a TextDirection.',
+        ),
+        ErrorHint(
+          'This error usually occurs when $target is used in a widget without '
+          'a Directionality ancestor.',
+        ),
+        ErrorHint(
+          'Typically, the Directionality widget is introduced by the MaterialApp '
+          'or WidgetsApp widget at the top of your application widget tree. It '
+          'determines the ambient reading direction and is used, for example, to '
+          'determine how to lay out text, how to interpret "start" and "end" '
+          'values, and to resolve EdgeInsetsDirectional, '
+          'AlignmentDirectional, and other *Directional objects.',
+        ),
+      ]);
+    }
+    return true;
+  }());
+  return true;
+}
@@ -357,6 +357,11 @@ bool debugCheckHasMediaQuery(BuildContext context) {
 /// hit.
 ///
 /// Does nothing if asserts are disabled. Always returns true.
+///
+/// See also:
+///
+///  * [debugCheckHasDirectionality], which is a similar, but more general
+///    painting-library level function.
 bool debugCheckHasDirectionality(
   BuildContext context, {
   String? why,
 
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/painting.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -330,6 +331,20 @@ void main() {
     expect(BorderRadiusDirectional.circular(15.0) % 10.0, BorderRadiusDirectional.circular(5.0));
   });
 
+  test('BorderRadiusDirectional.resolve with null throws detailed error', () {
+    const BorderRadiusDirectional borderRadius = BorderRadiusDirectional.all(Radius.circular(1.0));
+    expect(
+      () => borderRadius.resolve(null),
+      throwsA(
+        isFlutterError.having(
+          (FlutterError e) => e.message,
+          'message',
+          allOf(contains('No TextDirection found.'), contains('without a Directionality ancestor')),
+        ),
+      ),
+    );
+  });
+
   test('BorderRadiusDirectional.lerp() invariants', () {
     final BorderRadiusDirectional a = BorderRadiusDirectional.circular(10.0);
     final BorderRadiusDirectional b = BorderRadiusDirectional.circular(20.0);
 
@@ -732,6 +732,29 @@ void main() {
     },
   );
 
+  testWidgets(
+    'Container with BorderRadiusDirectional and no Directionality throws a detailed error',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        Container(
+          decoration: BoxDecoration(
+            border: Border.all(),
+            borderRadius: const BorderRadiusDirectional.all(Radius.circular(1.0)),
+          ),
+        ),
+      );
+
+      expect(
+        tester.takeException(),
+        isFlutterError.having(
+          (FlutterError e) => e.message,
+          'message',
+          allOf(contains('No TextDirection found.'), contains('without a Directionality ancestor')),
+        ),
+      );
+    },
+  );
+
   testWidgets('using clipBehaviour and shadow, should not clip the shadow', (
     WidgetTester tester,
   ) async {