Problems with Gridview correctly obtaining data, The ntext data type cannot be selected as DISTINCT because it is not comparable.

I'm having problems with Gridview correctly displaying data.  First I have a drop down list that lists all part numbers from a series of tables:

        SelectCommand="SELECT PART_NUMBER FROM Capacitors UNION SELECT PART_NUMBER FROM Connectors UNION SELECT PART_NUMBER FROM [Crystals and Oscillators] UNION SELECT PART_NUMBER FROM Diodes UNION SELECT PART_NUMBER FROM ICs UNION SELECT PART_NUMBER FROM Inductors UNION SELECT PART_NUMBER FROM Misc UNION SELECT PART_NUMBER FROM Relays UNION SELECT PART_NUMBER FROM Resistors UNION SELECT PART_NUMBER FROM Switches UNION SELECT PART_NUMBER FROM Transformers UNION SELECT PART_NUMBER FROM Transistors ORDER BY PART_NUMBER">

The drop down list works fine.  Next I have a Gridview table that should extract the part number and description fields from various tables using the part number selected in the  drop down list:

        SelectCommand="SELECT [PART_NUMBER], [Description] FROM [Capacitors] WHERE ([PART_NUMBER] = @PART_NUMBER) 
UNION 
SELECT [PART_NUMBER], [Description] FROM [Connectors] WHERE ([PART_NUMBER] = @PART_NUMBER) 
UNION 
SELECT [PART_NUMBER], [Description] FROM [Crystals and Oscillators] WHERE ([PART_NUMBER] = @PART_NUMBER) 
UNION 
SELECT [PART_NUMBER], [Description] FROM [Diodes] WHERE ([PART_NUMBER] = @PART_NUMBER) 
UNION 
SELECT [PART_NUMBER], [Description] FROM [ICs] WHERE ([PART_NUMBER] = @PART_NUMBER) 
UNION 
SELECT [PART_NUMBER], [Description] FROM [Inductors] WHERE ([PART_NUMBER] = @PART_NUMBER) 
UNION 
SELECT [PART_NUMBER], [Description] FROM [Misc] WHERE ([PART_NUMBER] = @PART_NUMBER) 
UNION 
SELECT [PART_NUMBER], [Description] FROM [Relays] WHERE ([PART_NUMBER] = @PART_NUMBER) 
UNION 
SELECT [PART_NUMBER], [Description] FROM [Resistors] WHERE ([PART_NUMBER] = @PART_NUMBER) 
UNION 
SELECT [PART_NUMBER], [Description] FROM [Switches] WHERE ([PART_NUMBER] = @PART_NUMBER) 
UNION 
SELECT [PART_NUMBER], [Description] FROM [Transformers] WHERE ([PART_NUMBER] = @PART_NUMBER) 
UNION 
SELECT [PART_NUMBER], [Description] FROM [Transistors] WHERE ([PART_NUMBER] = @PART_NUMBER)">

When the program runs I get "The ntext data type cannot be selected as DISTINCT because it is not comparable."  I simply don't understand where this error is coming from.  If I strip it down to looking at one table the problem goes away.  This should be extracting the Description field, not comparing it to anything.  Other than the addition of Description this is the same as the select command in the drop down list.  I'm completely lost on this one.

The asp section is below:

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" 
        DataKeyNames="PART_NUMBER" DataSourceID="SqlDataSource2"><Columns><asp:BoundField DataField="PART_NUMBER" HeaderText="PART_NUMBER" 
                ReadOnly="True" SortExpression="PART_NUMBER" /><asp:BoundField DataField="Description" HeaderText="Description" 
                SortExpression="Description" /></Columns></asp:GridView>

Thanks!