Specified cast is not valid

Hvad betyder ovenstående?

Fejlen kommer fra den kode:

DataGrid dlResults;
SqlConnection myConn = DBTools.OpenConnection();
SqlDataReader sqldr = DBTools.Query("SELECT overskrift,tekst from TestTable",myConn);
sqldr.Read();
dlResults.DataSource = sqldr;
dlResults.DataBind();
sqldr.Close();

DataTable table = new DataTable();
table = (DataTable)dlResults.DataSource; <<<< HER FEJLER KODEN
for (int i=0;i<table.Rows.Count;i++)
{
Response.Write("TEST");
}