TabBar throws assertion indicator != null || indicatorWeight > 0.0 despite a TabBarThemeData.indicator

Steps to Reproduce

When the tab indicator is supplied via TabBarThemeData.indicator (the idiomatic place for app-wide styling) and you set indicatorWeight: 0 on the TabBar widget to suppress the default underline weight, the constructor assertion throws — even though a valid indicator is present (from the theme).

The assertion in TabBar (packages/flutter/lib/src/material/tabs.dart, 3.44.4) is:

assert(indicator != null || (indicatorWeight > 0.0));

It only inspects the widget-level indicator parameter and is blind to TabBarThemeData.indicator.

Minimal repro (flutter create, replace lib/main.dart, run):

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) => MaterialApp(
        theme: ThemeData(
          tabBarTheme: const TabBarThemeData(
            indicator: BoxDecoration(
              color: Colors.red,
              borderRadius: BorderRadius.all(Radius.circular(22)),
            ),
            indicatorSize: TabBarIndicatorSize.tab,
          ),
        ),
        home: DefaultTabController(
          length: 2,
          child: Scaffold(
            appBar: AppBar(
              bottom: const TabBar(
                indicatorWeight: 0, // <-- throws, despite the themed indicator above
                tabs: [Tab(text: 'A'), Tab(text: 'B')],
              ),
            ),
            body: const TabBarView(
              children: [Center(child: Text('A')), Center(child: Text('B'))],
            ),
          ),
        ),
      );
}

Expected results

The TabBar builds. The themed indicator (red pill) is painted, and indicatorWeight: 0 is accepted because a valid indicator is provided by the theme — there is no default underline weight to add.

Actual results

Construction throws:

'package:flutter/src/material/tabs.dart': Failed assertion: line 1042 pos 16:
'indicator != null || (indicatorWeight > 0.0)': is not true.

The only workaround is to pass a non-zero epsilon weight (e.g. indicatorWeight: 0.0000001), because passing a widget-level indicator to satisfy the assert would override the themed indicator entirely (widget-level indicator takes precedence over TabBarThemeData.indicator), losing the app-wide styling. So there is no way to combine "indicator from theme" + "zero indicator weight" without a hack.

Notes / proposed fix

This is distinct from TabBar indicatorWeight is not ignored as documented #171951 (closed as a documentation clarification via Update TabBar documentation to clarify indicatorWeight behavior #184104). That issue was about indicatorWeight not being ignored when a widget-level custom indicator is supplied. This report is about the assertion not recognizing a theme-level indicator, which is still present in 3.44.4.
The assertion is a construction-time check and therefore cannot see the resolved theme (no BuildContext available in the constructor). A fix would be to either drop the assert, or move the validation to build time where TabBarTheme.of(context).indicator is known, so that a theme-supplied indicator satisfies the "indicator provided" condition.

Flutter Doctor

[✓] Flutter (Channel stable, 3.44.4, on Ubuntu 6.15.11-x64v3-xanmod1, locale en_DE.UTF-8)
    • Framework revision ad70ec4617 (2026-06-24), engine a10d8ac38d
[✓] Android toolchain - develop for Android devices (Android SDK version 37.0.0)
[✓] Chrome - develop for the web
[✓] Connected device (3 available)
[✓] Network resources

• No issues found!

(Please paste your own flutter doctor -v when filing.)

@qeepcologne

@qeepcologne Thanks for the detailed reproduction and the minimal sample. I was able to reproduce the issue on an Android emulator using the provided code. Routing this issue to team-design for further investigation. The following assertion is triggered:

'package:flutter/src/material/tabs.dart': Failed assertion:
'indicator != null || (indicatorWeight > 0.0)': is not true.

Thanks for the investigation. I think it makes sense to move this check to run-time.

@dkwingsmt

@dkwingsmt the issue with the assertion being made at run-time is that it makes the error quite ugly on the screen, which I think is not good for developer experience.

This is what I got after moving the assertion from the constructors to inside Decoration _getIndicator(TabBarIndicatorSize indicatorSize) method:

Decoration _getIndicator(TabBarIndicatorSize indicatorSize) {
  final ThemeData theme = Theme.of(context);
  final TabBarThemeData tabBarTheme = TabBarTheme.of(context);

  if (widget.indicator != null) {
    return widget.indicator!;
  }
  if (tabBarTheme.indicator != null) {
    return tabBarTheme.indicator!;
  }
  assert(
    widget.indicatorWeight > 0.0,
    'The indicatorWeight must be greater than 0 when the TabBar '
    'draws its default underline indicator (i.e. when no indicator is '
    'provided by the TabBar or the TabBarTheme).',
  );
    
  // Rest of the method
}

test: add regression tests for TabBar indicatorWeight: 0 with themed …

[material_ui] Fix `TabBar` assertion to allow `indicatorWeight: 0` wi…

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TabBar throws assertion `indicator != null || indicatorWeight > 0.0` despite a `TabBarThemeData.indicator` #188837

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TabBar throws assertion indicator != null || indicatorWeight > 0.0 despite a TabBarThemeData.indicator #188837

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suryaKommana2662 commented on Jul 1, 2026

dkwingsmt commented on Jul 1, 2026

Massinissa-Mouhoub commented on Jul 8, 2026

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TabBar throws assertion `indicator != null || indicatorWeight > 0.0` despite a `TabBarThemeData.indicator` #188837