Error BC30455 - What am I doing wrong?

[Modulus]

I have this code here:

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

        RefreshList()
    End Sub

    Private Sub RefreshList()
        Dim procs() As System.Diagnostics.Process = System.Diagnostics.Process.GetProcesses
        Dim f As String

        'as from @Visual Vincent's comment:
        ListBox1.BeginUpdate()
        ListBox1.Items.Clear()

        For Each proc As System.Diagnostics.Process In procs
            f = GetProcessFileName(proc)
            If f.Length > 0 Then
                ListBox1.Items.Add(f)
                ListBox1.Items.Add("MD5: " & GetMD5String(f))
                ListBox1.Items.Add(String.Empty)
            End If

        Next

        ListBox1.EndUpdate()

    End Sub

    Private Function GetProcessFileName(proc As System.Diagnostics.Process) As String
        Dim strRet As String = String.Empty

        Try
            strRet = proc.MainModule.FileName
        Catch ex As Exception
            ' This catch used to ignore "Access is denied" exception.
        End Try
        Return strRet
    End Function

    Private Function GetMD5String(ByVal strFilename As String) As String
        Dim cMD5 = System.Security.Cryptography.MD5.Create
        Dim bytHash As Byte()
        Dim sb As New System.Text.StringBuilder
        Dim scanbox As New TextBox
        scanbox.Text = My.Computer.FileSystem.ReadAllText("viruslist.txt").ToString

        Using cStream As New IO.FileStream(strFilename, IO.FileMode.Open, IO.FileAccess.Read, IO.FileShare.Read)

            bytHash = cMD5.ComputeHash(cStream)
        End Using

        For Each b In bytHash
            sb.Append(b.ToString("X2"))
        Next

        If scanbox.Text.Contains(sb.ToString) Then
            MessageBox.Show("Virus Detected!")
        End If

        Return sb.ToString



    End Function

    Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
        Timer1.Enabled = False 'Stops the timer
        RefreshList()
        GetMD5String()
        Timer1.Enabled = True
    End Sub

Under the GetMD5String() it's giving me error:

BC30455 Visual Basic AND VB.NET Argument not specified for parameter 'strFileName' of 'Private Function GetMD5String(ByVal strFilename As String) As String'.

What am I doing wrong?