Crystal Reports: Unable to connect: incorrect log on parameters

Logon failed. Details: crdb_adoplus : Object reference not set to an instance of an object. Error in File  {CDE24CC7-73A4-4842-AA8B-D8F552EF97E7}.rpt: Unable to connect: incorrect log on parameters

 i am using crystal report in my web application with parameter eventhough i hve passaed parameter value as

 if (drpdept.SelectedValue != "")

            {

                paramField.Name = "dept";

                paramDiscreteValue.Value = drpdept.SelectedItem.Text;

                paramField.CurrentValues.Add(paramDiscreteValue);

                paramFields.Add(paramField);

            }

but still itd asking for the value . i get a screen where by it asks me param vale