server error in asp.net

Hi,
 
I have a 2 types of datatables one is total data to be stored this is bind to my grid this is my parent datatable. And another one is child datatable , this having some data imported from my parent datatable this is stored in my viewstate.
What is my problem is when i try to bind my parent datatable to my grid it showing some error message . I don't no what's that error exactly.
 

protected void gdDocument_ItemDataBound(object sender, DataGridItemEventArgs e)
{
   if (e.Item.ItemType == ListItemType.Item || e.Item.ItemType == ListItemType.AlternatingItem)
   {
 
     string Doc_Id = ((Label)e.Item.FindControl("lblDrawing")).Text;
     DataTable dtDrawing = ViewState["dtDrawing"] as DataTable;
     DataRow[] drs=dtDrawing.Select("Dwg_No="+Doc_Id);   
   }
}

 
server error.

Min (20) must be less than or equal to max (-1) in a Range object. 

can any one help me to resolve this issue.