How do I handle gridview delete event

Why is my event handler class not fireing?
I have the following:

ASPX:
<asp:GridView ID="vwPatient" runat="server" OnRowDeleted="GridView_RowDeleted" AutoGenerateDeleteButton="True">

Code Behind:
protected void GridView_RowDeleted(Object src, GridViewDeletedEventArgs e)
{
            if (e.Exception == null)
            {
                if (e.AffectedRows == 1)
                {
                    lblInfoPatient.Visible = true;
                    lblInfoPatient.Text = "Record deleted successfully.";
                }
                else
                {
                    lblInfoPatient.Visible = true;
                    lblInfoPatient.Text = "An error occurred during the delete operation.";
                }
            }
            else
            {
                lblInfoPatient.Visible = true;
                lblInfoPatient.Text = e.Exception.Message;
                e.ExceptionHandled = true;
            }
}
       
But yet I receive the following error:

The GridView 'vwPatient' fired event RowDeleting which wasn't handled.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  

Stack Trace:


[HttpException (0x80004005): The GridView 'vwPatient' fired event RowDeleting which wasn't handled.]
   System.Web.UI.WebControls.GridView.OnRowDeleting(GridViewDeleteEventArgs e) +1494719
   System.Web.UI.WebControls.GridView.HandleDelete(GridViewRow row, Int32 rowIndex) +604
   System.Web.UI.WebControls.GridView.HandleEvent(EventArgs e, Boolean causesValidation, String validationGroup) +1134
   System.Web.UI.WebControls.GridView.RaisePostBackEvent(String eventArgument) +199
   System.Web.UI.WebControls.GridView.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +7
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +11
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +177
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1746

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