Optional Parameters in Stored Procedure Did Not Return Any Records

Optional Parameters in Stored Procedure Did Not Return Any Records.
Access 2002 (SP1) and SQL 2000 Server

I have an ADP form that have a combo box and a text box, and I wanted it to do the following:

1) Whenever a user select a value from a combo box, but leave the text box blank, then Access will    retrieve all of the records that meet the ONE criteria from the combo box.

2) Whenever a user put in a value from a text box, but leave the combo box blank, then Access will
    retrieve all of the records that meet the ONE criteria from the text box.

3) Whenever a user select a value from a combo box, and enter a value in the text box, then Access
    will retrieve all of the records that meet the TWO criteria - one from the combo box, and the
    other from a text box.

Currently, Access only retrieve records when I select a value from a combo box
and ignore the criteria of the text box value.

Example of Error:
combo box value: 100
text box value:  1
Result: Retrieve all records where the combo box value is 100 including records where the
            text box value is 1, 2, 3 or otherwise.

combo box value: 100
text box value:  BLANK
Result: Retrieve all records in the table, regardless of whether or not the combo box
           value is a 100.

////////////////////////////////////////

Form Name: frmParameters_2
Combo Box Name: cboParameter
Text Box Name:  Text_Au_Ord
Where: These 2 controls are located on the Form Header Of  the Form frmParameters_2

Form Name: frmParameters_2
Record Source: byroyalty_2
Input Parameters:
    @percentage int=Forms!frmParameters_2!cboParameter,
    @auord int=Forms!frmParameters_2!Text_Au_Ord

THE TWO PARAMETERS ABOVE ARE OPTIONAL PARAMETERS.
IF NO parameter are given (combo box and text box are blank), then all
records will be displayed on the form frmParameters_2.

Text Box: txtAuID
Where: On the Detail Section of The Form frmParameters_2

////////////////////////////////////////

CREATE PROCEDURE byroyalty_2 @percentage int, @auord int
AS
select au_id from titleauthor
where titleauthor.royaltyper = @percentage OR  @percentage IS NULL
AND titleauthor.au_ord = @auord OR @auord IS NULL
GO

/////////////////////////////////////////

Private Sub cboParameter_AfterUpdate()
    Me.Requery  
End Sub

Private Sub Text_Au_Ord_AfterUpdate()
    Me.Requery
End Sub