Dataset Blues

i don't know if this is the right forum for this question but here goes.  I have created a gridview to display a list of records retrieved by a SQL query.  Everytime I run it, I get the following error:

Object must implement IConvertible.

 

I thought it might have to do with the data I'm passing in the session string from the previous page.  Here is that code:Protected Sub cmdSubmit_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles cmdSubmit.Click

Session.Add("Shift", shShift.SelectedItem)Session.Add("Type", shType.SelectedItem)

Session.Add("SelDate", OccDate.SelectedDate)

Response.Redirect("SelectIncRep.aspx")

 

Here is the code that is supposed to execute and display the data on page_init:

 

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" DataKeyNames="RID" DataSourceID="SqlDataSource1" Width="1215px" BorderStyle="Groove" BorderWidth="4px">

<Columns>

<asp:BoundField DataField="RID" HeaderText="Record ID:" InsertVisible="False" ReadOnly="True"

SortExpression="RID" />

<asp:BoundField DataField="Date" HeaderText="Date:" SortExpression="Date" />

<asp:BoundField DataField="Shift_ID" HeaderText="Shift:" SortExpression="Shift_ID" />

<asp:BoundField DataField="Xref_ID" HeaderText="Type Of Occurence:" SortExpression="Xref_ID" />

<asp:BoundField DataField="Notes" HeaderText="Notes:" SortExpression="Notes" />

</Columns>

</asp:GridView>

<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:PerfDataConnectionString %>"

SelectCommand="SELECT [RID], [Notes], [Date], [Shift_ID], [Xref_ID] FROM [Safety_data] WHERE (([Shift_ID] = @Shift_ID) AND ([Xref_ID] = @Xref_ID) AND ([Date] = @Date))">

<SelectParameters>

<asp:SessionParameter Name="Shift_ID" SessionField="Shift_ID" Type="Int32" />

<asp:SessionParameter Name="Xref_ID" SessionField="Type" Type="Int32" />

<asp:SessionParameter Name="Date" SessionField="Date" Type="DateTime" />

</SelectParameters>

</asp:SqlDataSource>

 

What could be causing the issue?