[Solved] Stored Procedure not returning value based on parameters in ASP.Net using VB.Net

Hi,

I have a vb.net webform where the grid page have search options.

One option is to select the items from a dropdown list: Items: M Number or Surname, this will bring up a search box for user to type in surname or m number and then a search button. This should bring up the results to grid. 

 SQL stored procedure

ALTER PROCEDURE [dbo].[usp_GetRecordbySurnamePMRef]
(
    @LastName nvarchar(100) = NULL,
    @PMRef nvarchar(50) = NULL    
)
AS
BEGIN
    SET NOCOUNT ON;
    SELECT *
    FROM Ab
    WHERE (LastName = @LastName OR @LastName IS NULL)
    AND (PMRef = @PMRef OR @PMRef IS NULL) 
END
GO

the stored procedure does work if i put complete information such as Lastname or PMRef number (M Number) in full.

Eg: If the lastname is 'Smith' and i put Smith it brings the record back, but doesnt bring anything if i put 'Smi'.

Protected Sub btnsearch_Click(sender As Object, e As EventArgs) Handles btnsearch.Click
    Dim sql As String = "usp_GetRecordbySurnamePMRef"
    Try
        Using myConnection As New SqlConnection(ConfigurationManager.ConnectionStrings("Database_Connection_Lookups").ConnectionString)
            Dim myCommand As SqlCommand = New SqlCommand(sql, myConnection) 
            Dim RecordStatus As String = ddlGridfilter.SelectedValue
            Dim LastName As String = Me.txtsearch.Text
            Dim PMRef As String = Me.txtsearch.Text 
            myCommand.CommandType = CommandType.StoredProcedure
            If Not String.IsNullOrEmpty(txtsearch.Text) Then 
                myCommand.Parameters.AddWithValue("@LastName", txtsearch.Text)
            Else
                myCommand.Parameters.AddWithValue("@LastName", DBNull.Value)
            End If
            If Not String.IsNullOrEmpty(txtsearch.Text) Then
                myCommand.Parameters.AddWithValue("@PMRef", txtsearch.Text)
            Else
                myCommand.Parameters.AddWithValue("@PMRef", DBNull.Value)
            End If
            myConnection.Open()
            Using sda As New SqlDataAdapter
                myCommand.Connection = myConnection
                sda.SelectCommand = myCommand
                Using ds As New DataSet
                    sda.Fill(ds)
                    Me.GridView1.DataSource = ds
                    Me.GridView1.DataBind() 
                End Using 
            End Using 
        End Using
    Catch ex As Exception
        Throw
        'Response.Redirect("error.aspx")
    End Try
End Sub

Please advice. 

Many Thanks