Failing to style ComboBoxColumn for validation errors

I'm using Visual Studio 2010, and am attempting to display validation errors to the user for a ComboBoxColumn in a Datagrid.  ValidationStep=UpdatedValue.  Here's the column xaml:

<DataGridComboBoxColumn Header="Model" x:Name="ModelCBColumn" ><DataGridComboBoxColumn.SelectedValueBinding><Binding Path="Model" Mode="TwoWay" 
                                 ValidatesOnDataErrors="True" ><Binding.ValidationRules><local:ModelValidationRule ValidationStep="UpdatedValue" /></Binding.ValidationRules></Binding></DataGridComboBoxColumn.SelectedValueBinding><DataGridComboBoxColumn.CellStyle><Style TargetType="DataGridCell"><Style.Triggers><Trigger Property="Validation.HasError" Value="True"><Setter Property="Background" Value="Red"/><Setter Property="ToolTipService.ToolTip" 
                                  Value="{Binding RelativeSource={RelativeSource Self},
                                    Path=(Validation.Errors)[0].ErrorContent}"/></Trigger></Style.Triggers></Style></DataGridComboBoxColumn.CellStyle></DataGridComboBoxColumn>

I think the validation rule is being applied, because when there's an error the cell will have a thin red border and there's an exclamation point in the row header.  But this is not the style that I've defined above.  The background does not turn red, nor is there a tooltip of the validation message.  What's missing?