DataSource Controls :: Invalid Command Sent To ExecuteXmlReader Error While Executing Xmlreader?

when i am executing xmlreader in enterprise library 4.1, iam getting an error like below

error: Invalid command sent to ExecuteXmlReader. The command must return an Xml result.

my code:

SqlDatabase

db = (
cmd = db.GetSqlStringCommand(
xmlReader = db.ExecuteXmlReader(cmd);
db = null;DbCommand cmd =
null;XmlReader xmlReader =
null;SqlDatabase)DatabaseFactory.CreateDatabase();"select
* from emp");DataSet ds = db.ExecuteDataSet(cmd);