Conversion from type 'DBNull' to type 'String' is not valid!

GSun

I am trying to populate a Listbox control with values from an Oracle database11g xe. But it raises this error every time:

Conversion from type 'DBNull' to type 'String' is not valid! so I handled the nulls from the database level. The error stop popping up but the Listbox control still remain blank.

I used following code:

Dim G_conn As New OracleConnection(Form3.g_JUNODb) 
G_conn.Open()  
Dim go_cmd As New OracleCommand 
go_cmd.Connection = G_conn 
go_cmd.CommandText = "select nvl(description,'desc'),nvl(qty,'0'), nvl(selling_price,'1'),nvl(Amount,'1') from RICHCUT.current_sale1" 
go_cmd.CommandType = CommandType.Text  
Dim G_dataReader As OracleDataReader = go_cmd.ExecuteReader() 
While G_dataReader.Read() ListBox1.Items.Add(CStr(G_dataReader.Item("description")).PadRight(32) + "   N" + CStr(G_dataReader.Item("selling_price")) + "   (" + CStr(G_dataReader.Item("qty")) + ")   N" + CStr(G_dataReader.Item("amount")))  
End While  
G_dataReader.Dispose() 
go_cmd.Dispose()

I have used these lines before and the application is working well. Just copied the same application and this problem started to my greatest surprise! I did not even add the nvl bla bla bla to the original select statement that's working!I need urgent help please.Thanks in anticipation..