Microsoft EnterpriseLibrary 'connection

I hope this is the right place to ask this:

I am using the microsoft enterpriselibrary for accessing a sql server database.  when I use the following code, I receive an error 'ExecuteScalar: Connection property has not been initialized.':

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Database db = new SqlDatabase(p_ConnectionString);

DbCommand cmd = db.GetSqlStringCommand(p_Sql);

output = cmd.ExecuteScalar();

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but when I use this code, everything works correctly:

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Database db = new SqlDatabase(p_ConnectionString);

DbCommand cmd = db.GetSqlStringCommand(p_Sql);

output = db.ExecuteScalar(cmd);

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I was assured by my supervisor that both snippets' syntax are correct.  Can anyone shine some light on why the first snippet will throw an exception?

Thanks in advance,

Drew