diff --git a/examples/rendering/interactive_flex.dart b/examples/rendering/interactive_flex.dart
index 176a755ffc..d9c197925a 100644
--- a/examples/rendering/interactive_flex.dart
+++ b/examples/rendering/interactive_flex.dart
@@ -121,6 +121,6 @@ Pancetta meatball tongue tenderloin rump tail jowl boudin.""";
     child: row
   );
 
-  updateTaskDescription('Interactive Flex', topColor);
+  updateTaskDescription(label: 'Interactive Flex', color: topColor);
   new DemoBinding(root: root);
 }
diff --git a/packages/flutter/lib/src/services/activity.dart b/packages/flutter/lib/src/services/activity.dart
index af2f65b649..4c3feaee2e 100644
--- a/packages/flutter/lib/src/services/activity.dart
+++ b/packages/flutter/lib/src/services/activity.dart
@@ -11,7 +11,7 @@ import 'shell.dart';
 
 export 'package:sky_services/activity/activity.mojom.dart';
 
-/// Dart wrapper around Activity mojo service available in Sky on Android.
+/// Dart wrapper around Activity mojo service available in Flutter on Android.
 ///
 /// Most clients will want to use these methods instead of the activity service
 /// directly.
@@ -50,8 +50,10 @@ final PathService pathService = _pathServiceProxy.ptr;
 Color _cachedPrimaryColor;
 String _cachedLabel;
 
-/// Sets the TaskDescription for the current Activity
-void updateTaskDescription(String label, Color color) {
+/// Sets the TaskDescription for the current Activity.
+/// The color, if provided, must be opaque.
+void updateTaskDescription({ String label, Color color }) {
+  assert(color == null || color.alpha == 0xFF);
   if (_cachedPrimaryColor == color && _cachedLabel == label)
     return;
 
diff --git a/packages/flutter/lib/src/widgets/title.dart b/packages/flutter/lib/src/widgets/title.dart
index 2908a810d1..57edd8e1d1 100644
--- a/packages/flutter/lib/src/widgets/title.dart
+++ b/packages/flutter/lib/src/widgets/title.dart
@@ -7,7 +7,9 @@ import 'package:flutter/widgets.dart';
 
 /// Controls the description of this app in the operating system.
 class Title extends StatelessComponent {
-  Title({ this.title, this.child, this.color });
+  Title({ this.title, this.child, this.color }) {
+    assert(color == null || color.alpha == 0xFF);
+  }
 
   final Widget child;
 
@@ -18,13 +20,15 @@ class Title extends StatelessComponent {
   final Color color;
 
   Widget build(BuildContext context) {
-    updateTaskDescription(title, color);
+    updateTaskDescription(label: title, color: color);
     return child;
   }
 
   void debugFillDescription(List<String> description) {
     super.debugFillDescription(description);
-    description.add('"$title"');
-    description.add('color: $color');
+    if (title != null)
+      description.add('"$title"');
+    if (color != null)
+      description.add('color: $color');
   }
 }