Try this.
vb Code:
  1. Public Function GetNullBytes(ByRef lNum As Object) As String

  2.        Dim Buf As String = "" 

  3.         Dim i As Short

  4.         For i = 1 To lNum

  5.             Buf = Buf & Chr(0)

  6.         Next

  7.         GetNullBytes = Buf

  8.     End Function