Made changes requested by @grokys.

Avalonia.sln

@@ -1,6 +1,6 @@
 Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 14
-VisualStudioVersion = 14.0.25420.1
+# Visual Studio 15
+VisualStudioVersion = 15.0.26403.3
 MinimumVisualStudioVersion = 10.0.40219.1
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Avalonia.Base", "src\Avalonia.Base\Avalonia.Base.csproj", "{B09B78D8-9B26-48B0-9149-D64A2F120F3F}"
 EndProject
@@ -176,6 +176,7 @@ Global
 		tests\Avalonia.RenderTests\Avalonia.RenderTests.projitems*{48840edd-24bf-495d-911e-2eb12ae75d3b}*SharedItemsImports = 13
 		src\Shared\PlatformSupport\PlatformSupport.projitems*{4a1abb09-9047-4bd5-a4ad-a055e52c5ee0}*SharedItemsImports = 4
 		src\Shared\PlatformSupport\PlatformSupport.projitems*{7863ea94-f0fb-4386-bf8c-e5bfa761560a}*SharedItemsImports = 4
+		src\Shared\PlatformSupport\PlatformSupport.projitems*{7b92af71-6287-4693-9dcb-bd5b6e927e23}*SharedItemsImports = 4
 		src\Shared\RenderHelpers\RenderHelpers.projitems*{7d2d3083-71dd-4cc9-8907-39a0d86fb322}*SharedItemsImports = 4
 		src\Skia\Avalonia.Skia\Avalonia.Skia.projitems*{7d2d3083-71dd-4cc9-8907-39a0d86fb322}*SharedItemsImports = 4
 		src\Windows\Avalonia.Win32\Avalonia.Win32.Shared.projitems*{811a76cf-1cf6-440f-963b-bbe31bd72a82}*SharedItemsImports = 4

src/Avalonia.Controls/Button.cs

@@ -249,7 +249,10 @@ namespace Avalonia.Controls
             base.UpdateDataValidation(property, status);
             if(property == CommandProperty)
             {
-                IsEnabled = status?.ErrorType != BindingErrorType.Error;
+                if(status?.ErrorType == BindingErrorType.Error)
+                {
+                    IsEnabled = false;
+                }
             }
         }
 

src/Markup/Avalonia.Markup/Data/ExpressionObserver.cs

@@ -239,17 +239,7 @@ namespace Avalonia.Markup.Data
 
                 if (broken != null)
                 {
-                    // We've received notification of a broken expression due to a null value
-                    // somewhere in the chain. If this null value occurs at the first node then we
-                    // ignore it, as its likely that e.g. the DataContext has not yet been set up.
-                    if (broken.HasNodes)
-                    {
-                        broken.Commit(Description);
-                    }
-                    else
-                    {
-                        o = AvaloniaProperty.UnsetValue;
-                    }
+                    broken.Commit(Description);
                 }
                 return o;
             }

tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj

@@ -95,6 +95,7 @@
   </Choose>
   <ItemGroup>
     <Compile Include="AppBuilderTests.cs" />
+    <Compile Include="ButtonTests.cs" />
     <Compile Include="CanvasTests.cs" />
     <Compile Include="ClassesTests.cs" />
     <Compile Include="LayoutTransformControlTests.cs" />

tests/Avalonia.Controls.UnitTests/ButtonTests.cs

@@ -0,0 +1,125 @@
+using System;
+using System.Windows.Input;
+using Avalonia.Markup.Xaml.Data;
+using Xunit;
+
+namespace Avalonia.Controls.UnitTests
+{
+    public class ButtonTests
+    {
+        [Fact]
+        public void Button_Is_Disabled_When_Command_Is_Disabled()
+        {
+            var command = new TestCommand(false);
+            var target = new Button
+            {
+                Command = command,
+            };
+
+            Assert.False(target.IsEnabled);
+            command.IsEnabled = true;
+            Assert.True(target.IsEnabled);
+            command.IsEnabled = false;
+            Assert.False(target.IsEnabled);
+        }
+
+        [Fact]
+        public void Button_Is_Disabled_When_Bound_Command_Doesnt_Exist()
+        {
+            var target = new Button
+            {
+                [!Button.CommandProperty] = new Binding("Command"),
+            };
+
+            Assert.False(target.IsEnabled);
+        }
+
+        [Fact]
+        public void Button_Is_Disabled_When_Bound_Command_Is_Removed()
+        {
+            var viewModel = new
+            {
+                Command = new TestCommand(true),
+            };
+
+            var target = new Button
+            {
+                DataContext = viewModel,
+                [!Button.CommandProperty] = new Binding("Command"),
+            };
+
+            Assert.True(target.IsEnabled);
+            target.DataContext = null;
+            Assert.False(target.IsEnabled);
+        }
+
+        [Fact]
+        public void Button_Is_Enabled_When_Bound_Command_Is_Added()
+        {
+            var viewModel = new
+            {
+                Command = new TestCommand(true),
+            };
+
+            var target = new Button
+            {
+                DataContext = new object(),
+                [!Button.CommandProperty] = new Binding("Command"),
+            };
+
+            Assert.False(target.IsEnabled);
+            target.DataContext = viewModel;
+            Assert.True(target.IsEnabled);
+        }
+
+        [Fact]
+        public void Button_Is_Disabled_When_Disabled_Bound_Command_Is_Added()
+        {
+            var viewModel = new
+            {
+                Command = new TestCommand(false),
+            };
+
+            var target = new Button
+            {
+                DataContext = new object(),
+                [!Button.CommandProperty] = new Binding("Command"),
+            };
+
+            Assert.False(target.IsEnabled);
+            target.DataContext = viewModel;
+            Assert.False(target.IsEnabled);
+        }
+
+        private class TestCommand : ICommand
+        {
+            private bool _enabled;
+
+            public TestCommand(bool enabled)
+            {
+                _enabled = enabled;
+            }
+
+            public bool IsEnabled
+            {
+                get { return _enabled; }
+                set
+                {
+                    if (_enabled != value)
+                    {
+                        _enabled = value;
+                        CanExecuteChanged?.Invoke(this, EventArgs.Empty);
+                    }
+                }
+            }
+
+            public event EventHandler CanExecuteChanged;
+
+            public bool CanExecute(object parameter) => _enabled;
+
+            public void Execute(object parameter)
+            {
+            }
+        }
+    }
+}