SqlDataSource Parameter Binding (Updating)

Hello, I just started working with ASP.NET.
I'm trying to use the CheckBoxList Control.  As I understand it, you can bind a SqlDataSource to this control and it loads the list for you.  However tp precheck the items, you have to do this manually.  This part works fine.  Next part was to save whatever the user checks.  I wrote stored Procedure and now just trying to pass 1 parameter to the stored procedure using a second SqlDataSource.  I get the error: "A severe error occurred on the current command.  The results, if any, should be discarded."
Here is the second datasource I am using to try and save the data:
<asp:SqlDataSource ID="sds_PersonRole" runat="server" ConnectionString="<%$ ConnectionStrings:Development %>"
SelectCommand="usp_selectPersonRole"
SelectCommandType="StoredProcedure"
UpdateCommand="usp_updatePersonRole"
UpdateCommandType="StoredProcedure">
<SelectParameters>
    <asp:ControlParameter ControlID="gv_Person" Name="PERSON_ID" PropertyName="SelectedValue" Type="Int32" />
</SelectParameters>
<UpdateParameters>
    <asp:Parameter Name="strXML" Size="8000" Type="String" />
    <asp:Parameter Direction="InputOutput" Name="err_msg" Type="String" Size="150" DefaultValue="0" />
</UpdateParameters>
</asp:SqlDataSource>
I have a code behind file with the following modules:  (btnEditPerson is clicked to start the process.  the sds_PersonDetails is updated via form contolls and works fine.)  Error occurs on the bolded line (Stored procedure is just accepting the string and saving to a field.  It works fine, I tested it.  Its just erroring out before  it runs the stored procedure.
Protected Sub btnEditPerson_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnEditPerson.Click
    sds_PersonDetails.Update()
    gv_Person.DataBind()

    sds_PersonRole.Update()
End Sub
 
    Protected Sub sds_PersonRole_Updating(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.SqlDataSourceCommandEventArgs) Handles sds_PersonRole.Updating
        Dim command As Data.Common.DbCommand
        command = e.Command
        'un-check all checkboxlist items (count - 1 to account for starting at 0)
        Dim listCount As Integer = cbl_Role.Items.Count() - 1
        Dim strXML As String
        strXML = "<personRole>"
        For x As Integer = 0 To listCount
            If cbl_Role.Items(x).Selected() = False Then
                strXML = strXML & "<person id='" & gv_Person.SelectedValue & "' />"
                strXML = strXML & "<role id='" & cbl_Role.Items(x).Value & "' />"
            End If
        Next
        strXML = strXML & "</personRole>"

        command.Parameters("@strXML").Value = strXML
        lbl_ErrMsg.Text = command.Parameters("@err_msg").Value.ToString()
    End Sub