Weird XAML parsing error when trying to set TextBox.IsReadOnly

I've managed to reduce this to a simple test case. An exception is thrown during the parsing of this XAML using XamlReader.Parse():

<DockPanel xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <DockPanel.Resources>
        <Style TargetType="TextBox">
            <Style.Triggers>
                <Trigger Property="IsReadOnly" Value="True">
                    <Setter Property="Background" Value="#FFEEEEEE" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </DockPanel.Resources>


    <TextBox IsReadOnly="True" />
</DockPanel>

The exception message is:

Cannot set unknown member 'System.Windows.Controls.TextBox.IsReadOnly'. Line number '13' and line position '11'.

If I don't set IsReadOnly on the TextBox, it parses fine. It also parses fine if I remove the style trigger.

Can anyone shed some light on this? I'm rather new to WPF.

UPDATE:
Here's the unit test I'm using to reproduce this (it's failing on my PC):

[TestMethod]
public void TestIsReadOnlyOnTextBox()
{
    // Arrange
    var xaml =
@"<DockPanel xmlns=""http://schemas.microsoft.com/winfx/2006/xaml/presentation"" xmlns:x=""http://schemas.microsoft.com/winfx/2006/xaml"">
    <DockPanel.Resources>
        <Style TargetType=""TextBox"">
            <Style.Triggers>
                <Trigger Property=""IsReadOnly"" Value=""True"">
                    <Setter Property=""Background"" Value=""#FFEEEEEE"" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </DockPanel.Resources>


    <TextBox IsReadOnly=""True"" />
</DockPanel>
";

    // Act
    try {
        var root = XamlReader.Parse(xaml);
    }
    catch (XamlParseException ex) {
        Assert.Fail(ex.Message);
    }

    // If we get here, test passes
}

UPDATE 2:
I was originally referencing just PresentationFramework v4.0.30319. Adding references to PresentationCore, System.Xaml, and WindowsBase has no effect.

.NET version of project is 4 (full, not client profile).

UPDATE 3:
Arg, this works fine in ExpressionBlend 3.0.1927.0 and XamlPadX 4. As reported by AresAvatar, it seems to only fail when parsed with XamlReader.Parse() or XamlReader.Load()!