“Prefix 'x' does not map to a namespace”

I want to load a DataTemplate at runtime using XamlReader, but it's throwing the exception "Prefix 'x' does not map to a namespace."

This is the XML string I'm passing to XamlReader:

<xm:ResourceDictionary 
    xmlns:xm="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:do="clr-namespace:MyLibrary.DataObjects;assembly=MyLibrary.DataObjects"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
      <xm:DataTemplate DataType="{x:Type do:ValidationResponse}">
        <xm:StackPanel Orientation="Horizontal">
          <xm:Label>MessageID</xm:Label>
          <xm:TextBox Text="{Binding Path=MessageID}"/>
        </xm:StackPanel>
      </xm:DataTemplate>
</xm:ResourceDictionary>

This is the code that's reading it:

ResourceDictionary dictionary = XamlReader.Parse(myXamlString) as ResourceDictionary;

Here's the funny part, if I add x:Key="ValidationResponseTemplate" to the DataTemplate it parses without any exceptions. I can't keep it that way, however, because I can't specify the DataTemplate by key in the program's own .xaml (it won't know about the template until it gets fetched at runtime).

The x namespace is defined in both the program's own .xaml and in the fragment of XML I'm trying to parse.

Overall objective: be able to provide new DataTemplates to both change the appearance of the display at runtime, and to display XML data that the client did not know about at compile-time.