Unable to cast object of type 'ASP.usercontrols_dashboard_ucmytasks_ascx' to type 'System.Web.UI.WebControls.GridViewRow'.

I am using GridView in user controls.  In exisiting code we have 9 columns which doesn't have any sort expression.  As per new requirements we need to add Sorting functionality for just 2 columns like project date & project code ( which is alphanemuric).  So i have added like below for these 2 columns.
<asp:TemplateField HeaderText="Project Start Date" SortExpression="Project_Start_Date" ItemStyle-Width="110px">
<ItemTemplate>
<asp:Label ID="Start_Date" Text='<%#Eval("Project_Start_Date", "{0:MM/dd/yyyy}")%>' runat="server" CommandArgument="<%#((GridViewRow)Container).RowIndex%>" CssClass="lblCapitalize" />
</ItemTemplate>
</asp:TemplateField>
Now after adding sortexpression for these column and when i click lablel Project date, i am getting error like
"Input string is n ...