DataSource does not support sorting

I added a radiobuttonlist to the page.

<asp:RadioButtonList ID="Pending" runat="server" AutoPostBack="True" 
                     OnSelectedIndexChanged="Populategrid" Height="16px" 
                     RepeatDirection="Horizontal" Width="755px">
                     <asp:ListItem Value="250">Pending Over 250</asp:ListItem>
                     <asp:ListItem Value="300">Pending Over 300</asp:ListItem>
                     <asp:ListItem Value="350" Selected="True">Pending Over 350</asp:ListItem>
                     <asp:ListItem Value="400">Pending Over 400</asp:ListItem>

                 </asp:RadioButtonList>

I have sorting turned on in the gridview:

  <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"   
        DataSourceID="SqlDataSource1" EnableModelValidation="True" Width="789px" 
        CaptionAlign="Top" Height="126px" BackColor="White" BorderColor="#DCBE68" 
             BorderStyle="Solid" BorderWidth="1px" CellPadding="4" 
              HorizontalAlign="Center" CellSpacing="2" Font-Names="Arial" 
            AllowSorting="True" EmptyDataText="No Records" DataKeyNames="FO,CLMSSN">
        <Columns>
              <asp:BoundField DataField="DOC" HeaderText="DOC" SortExpression="DOC" />
              <asp:BoundField DataField="RegionAcronym" HeaderText="Region" SortExpression="RegionAcronym" />
              <asp:BoundField DataField="FO" HeaderText="FO" SortExpression="FO" />
              <asp:BoundField DataField="CLMSSN" HeaderText="CLMSSN" 
                  SortExpression="CLMSSN" />
              <asp:BoundField DataField="Age" HeaderText="Age" ReadOnly="True" 
                  SortExpression="Age" />
              <asp:BoundField DataField="FileDate" HeaderText="FileDate" ReadOnly="True" 
                  SortExpression="FileDate" DataFormatString="{0:d}" />
              <asp:BoundField DataField="DDS" HeaderText="DDS" SortExpression="DDS" />
              <asp:BoundField DataField="DDSCode" HeaderText="DDSCode" 
                  SortExpression="DDSCode" />
              <asp:BoundField DataField="DDSReceipt" HeaderText="DDSReceipt" ReadOnly="True" 
                  SortExpression="DDSReceipt" DataFormatString="{0:d}" />
              <asp:BoundField DataField="DDSAge" HeaderText="DDSAge" ReadOnly="True" 
                  SortExpression="DDSAge" />
              <asp:BoundField DataField="Title" HeaderText="Title" ReadOnly="True" 
                  SortExpression="Title" />    </Columns>
        <FooterStyle BackColor="#FFFFCC" ForeColor="#330099" />
        <HeaderStyle BackColor="#990000" Font-Bold="True" ForeColor="#FFFFCC" 
            VerticalAlign="Top" Font-Size="Medium" />
        <PagerStyle BackColor="#FFFFCC" ForeColor="#330099" HorizontalAlign="Center" />
        <RowStyle BackColor="White" BorderStyle="Double" ForeColor="#330099" 
                HorizontalAlign="Center" />
        <SelectedRowStyle BackColor="#FFCC66" Font-Bold="True" ForeColor="#663399" />
    </asp:GridView>

When I try to sort in the gridview then go and select something from the radiobuttonlist again I am getting the 

following error:

DataSource does not support sorting.

I'm getting it on Gridview1.DataBind()

 If Pending.SelectedValue = "400" Then
            Dim conn As New Data.SqlClient.SqlConnection(ConfigurationManager.ConnectionStrings("PendingClaimsConnectionString").ConnectionString)
            Dim cmd As New Data.SqlClient.SqlCommand
            Dim cmdPending As New Data.SqlClient.SqlCommand
            With cmdPending
                .CommandType = Data.CommandType.StoredProcedure
                .CommandText = "GetDDS400"                .Connection = conn
                .Parameters.AddWithValue("@doc", Request.QueryString("doc"))
            End With
            Dim adapter As New Data.SqlClient.SqlDataAdapter(cmdPending)
            Try                conn.Open()
                Dim ds As New Data.DataSet                adapter.Fill(ds)
                GridView1.DataSourceID = ""
                If GridView1.EmptyDataText = "" Then
                    Response.Write("No Pending Records")                End If
                GridView1.DataSource = ds                GridView1.DataBind()
            Finally                conn.Dispose()            End Try
        End If    End Sub

Do I need to handle the sorting a different way since I'm using a datalist?