VS Express 2013 Invalid Markup with OxyPlot

I'm developing an application in VS Express 2013 using C#/XAML and referencing the pre-built OxyPlot binaries (Express doesn't support PCL). Everything works just fine, except when I add a plot to the XAML code. The Design window states there's an invalid markup, but the project compiles and runs just fine, with the charts working.

Here's the XAML code in question. The oxy:Plot controls are presenting a warning: "The name "Plot" does not exist in the namespace "http://oxyplot.codeplex.com"". Any idea how to fix this issue?

    <Window x:Class="Playback.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:oxy="http://oxyplot.codeplex.com"
        xmlns:local="clr-namespace:Playback"
        Title="Playback" Height="700" Width="1260" WindowStartupLocation="CenterScreen">
    <Window.DataContext>
        <local:MainViewModel />
    </Window.DataContext>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
            <RowDefinition/>
            <RowDefinition Height="65"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="201*"/>
            <ColumnDefinition Width="425*"/>
        </Grid.ColumnDefinitions>
        <oxy:Plot Model="{Binding UpChart}" Margin="0,20,0,0" Grid.Column="0" Grid.Row="0"/>
        <oxy:Plot Model="{Binding MidChart}" Margin="0,20,0,0" Grid.Column="0" Grid.Row="1"/>
        <oxy:Plot Model="{Binding LowChart}" Margin="0,20,0,0" Grid.Column="0" Grid.Row="2"/>
    </Grid>
</Window>

Thank you!!

Sorry, I don't know what is wrong here.
You could try to build a .NET 4 or 4.5 version of the core library (there is a .NET 3.5 project you can start from) and see if the errors disappear.