Problem in converting LINQ result set to datatable

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ASP.NET

LINQ

Hello friends
i have to convert my linq result set to datatable
for this i am using the code

char HeadType = _session.HeadType;
     var res = db.DispHed(_session.CompId;
     DataView dv = _global.ToDataTable(db, res).DefaultView;

and ToDataTable methos in class file is

public DataTable ToDataTable(System.Data.Linq.DataContext ctx, object query) 
    {
        DataTable dt = new DataTable();
        if (query != null)
        {
            IDbCommand cmd = ctx.GetCommand(query as IQueryable);
            SqlDataAdapter adapter = new SqlDataAdapter();
            adapter.SelectCommand = (SqlCommand)cmd;
            try
            {
                cmd.Connection.Open();
                adapter.FillSchema(dt, SchemaType.Source);
                adapter.Fill(dt);
            }
            finally
            {
                cmd.Connection.Close();
            }
        }
        return dt; 
    } 

but on reaching line
IDbCommand cmd = ctx.GetCommand(query as IQueryable);
it is giving error

Value cannot be null.
Parameter name: query

thanks in advance