Issue With SQLDataSource And FilterExpression

I'm not sure exactly how the FilterExpression works. I have a sqldatasource, stored procedure, and GridView. My stored procedure basicly a select statement to populate my gridview, it included the fields I want to filter on. In my codebehind file I build a WHERE Clause based on the entries a user makes. Then I add the my FilterExpr variable to the SqlDataSource1.FilterExpression = FilterExpr. My SqlDataSource has a number of control parameters that match the textboxes a users enters into.

My question, I guess is does my stored procedure need the variables matching my controlparameters for my sqldatasource? Or how does this work? My GridView is returning all rows no matter what I enter into the filter textboxes (first, last, etc...)

MY SQLSDATASOURCE <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
SelectCommand="ClientSearch" SelectCommandType="StoredProcedure">
<FilterParameters>
<asp:ControlParameter ControlID="SearchLastName" Name="LastName" PropertyName="Text" ConvertEmptyStringToNull="true" />
<asp:ControlParameter ControlID="SearchFirstName" Name="FirstName" PropertyName="Text" ConvertEmptyStringToNull="true" />
<asp:ControlParameter ControlID="SearchEmail" Name="Email" PropertyName="Text" ConvertEmptyStringToNull="true" />
<asp:ControlParameter ControlID="SearchAddress" Name="Address" PropertyName="Text" ConvertEmptyStringToNull="true" />
<asp:ControlParameter ControlID="SearchComment" Name="Comment" PropertyName="Text" ConvertEmptyStringToNull="true" />
</FilterParameters>
</asp:SqlDataSource>MY CODEBEHIND
        Dim FilterExpr As String

        If SearchLastName.Text = "" And _
            SearchFirstName.Text = "" And _
            SearchEmail.Text = "" And _
            SearchComment.Text = "" And _
            SearchAddress.Text = "" Then
            lblMessage.Text = "You didn't enter any search parameters. Please try Again."
            Me.GridViewSearch.DataSourceID = ""
            Exit Sub
        Else
            Me.GridViewSearch.DataSourceID = "SqlDataSource1"
        End If

        FilterExpr = ""
        If SearchLastName.Text <> "" Then
            FilterExpr = FilterExpr & "LastName Like '" & _
            SearchLastName.Text & "%" & "' AND "
        End If

        If SearchFirstName.Text <> "" Then
            FilterExpr = FilterExpr & "FirstName Like '" & _
            SearchFirstName.Text & "%" & "' AND "
        End If
        If SearchEmail.Text <> "" Then
            FilterExpr = FilterExpr & " Like '" & _
             SearchEmail.Text & "%" & "' AND "
        End If
        If SearchComment.Text <> "" Then
            FilterExpr = FilterExpr & "[Comments] Like '" & "%" & _
             SearchComment.Text & "%" & "' AND "
        End If
        If SearchAddress.Text <> "" Then
            FilterExpr = FilterExpr & "[Address] Like '" & "%" & _
             SearchAddress.Text & "%" & "' AND "
        End If

        If Right(FilterExpr, 4) = "AND " Then
            FilterExpr = Left(FilterExpr, Len(FilterExpr) - 4)
        End If
        Try
            Me.SqlDataSource1.FilterExpression = FilterExpr
            Me.SqlDataSource1.DataBind()
            Me.GridViewSearch.DataBind()
            Me.lblMessage.Text = Me.SqlDataSource1.FilterExpression
        Catch objException As SqlException
            Dim objError As SqlError
            For Each objError In objException.Errors
                Response.Write(objError.Message)
            Next
        End Try
    End Sub 

MY SPROCGO
ALTER PROCEDURE [dbo].[ClientSearch]

AS
SELECT C.ClientID, C.FirstName, C.LastName, A.Address,
C.Comments, C.EMail
FROM tblClient C INNER JOIN
tblClientAddresses A ON C.ClientID = A.ClientID