C# – Sorting a GridView with an ObjectDataSource is not Sorting

I have a gridview like below:

 <asp:GridView DataKeyNames="TransactionID" 
               AllowSorting="True" AllowPaging="True"ID="grvBrokerage" 
               runat="server" AutoGenerateColumns="False" 
               CssClass="datatable" Width="100%"
              <Columns>
                  <asp:BoundField DataField="BrkgAccountNameOutput"              
                                  HeaderText="Account Name"/>
                  <asp:BoundField DataField="TransactionAmount" 
                                  HeaderText="Transaction Amount" 
                                  SortExpression="TransactionAmount" />
                  <asp:BoundField DataField="TransType" 
                                  HeaderText="Transaction Type"          
                                  SortExpression="TransType"/>
                  <asp:BoundField DataField="AccountBalance" 
                                  HeaderText="Account Balance"/>
                  <asp:BoundField DataField="CreateDt" 
                                  HeaderText="Transaction Date"  />
               </Columns>
 </asp:GridView>

I have a page with a gridview and a objectdatasource control. AllowPaging and AllowSorting is enabled. Here is the method I use that gets the data and binds the objectdatasource to the grid:

        protected void BindBrokerageDetails()
        {
            HomePage master = (HomePage)Page.Master;

            BrokerageAccount brokerageAccount = new BrokerageAccount();
            brokerageAccount.UserID = new Guid(Membership.GetUser().ProviderUserKey.ToString()); 

            ddlBrokerageDetails.DataSource = brokerageAccount.GetAll();
            ddlBrokerageDetails.DataTextField = "Account Name";
            ddlBrokerageDetails.DataValueField = "Account Name";
            ddlBrokerageDetails.DataBind();
           if (ddlBrokerageDetails.Items.Count > 0)
           {
               BrokerageTransactions brokerageaccountdetails = new          
                                           BrokerageTransactions();
               DataSet ds = BrokerageAccount.GetBrkID2(
                                    new Guid(Membership
                                              .GetUser()
                                              .ProviderUserKey
                                              .ToString()),
                                    ddlBrokerageDetails
                                              .SelectedItem
                                              .Text
                                              .ToString());
               foreach (DataRow dr in ds.Tables[0].Rows)
               {
                   brokerageaccountdetails.BrokerageId = new Guid(dr["BrkrgId"].ToString());
               }

               ddlBrokerageDetails.SelectedItem.Value = brokerageaccountdetails.BrokerageId.ToString();
               grvBrokerage.DataSource = ObjectDataSource1;
               grvBrokerage.DataBind();
           }           
      }

I have a sorting event, but when I check the grvBrokerage.DataSource, it is null. I am curious as to why? Here is the code for that?

  protected void grvBrokerage_Sorting(object sender, GridViewSortEventArgs e)
       {                
         DataTable dt = grvBrokerage.DataSource as DataTable;

            if (dt != null)
            {                    
              DataView dv = new DataView(dt);
              dv.Sort = e.SortExpression + " " + e.SortDirection;    
              grvBrokerage.DataSource = dv;
              grvBrokerage.DataBind();
           }
       }

Here is the ObjectDataSource declaration:

<asp:ObjectDataSource ID="ObjectDataSource1" 
                      runat="server" 
                      SelectMethod="GetAllWithId"
                      TypeName="BrokerageTransactions">
                      <SelectParameters>
                          <asp:ControlParameter  
                              ControlID="ddlBrokerageDetails"           
                              Name="brokid" 
                              PropertyName="SelectedValue"
                              Type="Object" />
                      </SelectParameters>
</asp:ObjectDataSource>

Thanks,
X