How To Pass NULL To SQLdatasource Stored Param

I'm developing a web app using VS2005.  I have a webpage with panel containing a gridview populated by a SQLdatasource.  The SQLdatasource in turn is populated by a stored procedure that can take up to 5 parameters.  The user types in up to 5 separate words (searchterms) in a text box which are then parsed and passed to the stored proc in the datasource which performs a fulltext search.  The gridview then becomes visible.  My problem is that unless the user types in 5 searchterms (no less), the gridview returns zero rows.  5 searchterms returns the proper results.  Somehow, I need to be able to pass in null or empty values for unneeded parameters.

I've tested the stored procedure in Query Analyzer and from within the SQLdatasource configuration (using Test Query) using 0 up to 5 parameters and it works fine, so that's not my problem.  Here's the code that runs after the user types in their search term(s) and presses a button:Public Sub FTSearch_Command(ByVal sender As Object, ByVal e As CommandEventArgs) Handles btnFullText.Command

Dim x As Integer

pnlFullText.Visible = Falsefiltertext = Replace(txtSearchTxt.Text, "'", "''")

If Not filtertext Is Nothing Then

filtertext = filtertext.Trim

Else

Return

End IfDim arrayString() As String = filtertext.Split(" ")

Dim length As Integer = arrayString.LengthFor x = 0 To (length - 1)

If Not arrayString(x) Is Nothing ThenSelect Case x

Case 0 : lblFTParm1.Text = arrayString(0)Case 1 : lblFTParm2.Text = arrayString(1)

Case 2 : lblFTParm3.Text = arrayString(2)Case 3 : lblFTParm4.Text = arrayString(3)

Case 4 : lblFTParm5.Text = arrayString(4)

End Select

End If

Next

pnlFullText.Visible = "True"

End Sub

Any ideas? 

Thanks in advance.