LINQ .Startswith或.Contains VB.NET4中的问题

在我的代码中,我可以轻松地使用“where Obj.Feld = String”,但使用“where Obj.Feld.StartsWith(”a“)”不起作用.请参阅以下两个功能：

Public Function EntriesByFileName(ByRef Database() As Entry,ByVal Filename As _
  String) As IEnumerable(Of Entry)
        Dim Result As IEnumerable(Of Entry) = From EntryObject In Database _
        Where (EntryObject.FileName = Filename) Select EntryObject
        Return Result
    End Function

    Public Function EntriesLikeFileName(ByRef Database() As Entry,ByVal _
      Filename As String) As IEnumerable(Of Entry)
        Filename = Filename.ToLower
        Dim Result As IEnumerable(Of Entry) = From EntryObject In Database _
          Where EntryObject.FileName.StartsWith("a") Select EntryObject
        Return Result
    End Function

第一个函数(byFileName)工作正常.第二个函数(LikeFileName)没有.使用Startswith我得到“对象引用未设置为对象的实例”.我究竟做错了什么？

数据库是一个对象数组,一个由字符串组成的结构