Empty data cells when AutoPopulateColumns is False

We have a Silverlight DXGrid control bound to a RIA / Entity Framework data source within a MVVM setup.

Using
<dxg:GridControl
    x:Name="Grid"
    DataSource="{Binding Wakeups, Mode=OneWay}"
    Visibility="Visible"
    AutoPopulateColumns="True">
   
    <dxg:GridControl.View>
        <dxg:TableView AllowEditing="False" MultiSelectMode="Row" NavigationStyle="Row"/>
    </dxg:GridControl.View>
   
</dxg:GridControl>

we get the grid filled with all the data with the columns in alphabetical order.

However when we try to pick out particular columns using:

<dxg:GridControl
     x:Name="Grid"
     DataSource="{Binding Wakeups, Mode=OneWay}"
     Visibility="Visible">

     <dxg:GridControl.Columns>
        <dxg:GridColumn FieldName="Number" />
    </dxg:GridControl.Columns>
   
    <dxg:GridControl.View>
        <dxg:TableView AllowEditing="False" MultiSelectMode="Row" NavigationStyle="Row"/>
    </dxg:GridControl.View>
</dxg:GridControl>

we get the corresponding number of rows with the correct header row but all the data cells are empty.