Problem encounter in VB AsynRead

I am developing a actice-x control which can perform downloading  . But everytime I get this error message : A Property or method call cannot include a reference to a private object, either as a argument or as a return value .

Below is my code :

Command1_Click()
   a = SourcePath
   SourcePath = a
End Sub

Public Property Get SourcePath() As String
    SourcePath = m_SourcePath
End Property

Public Property Let SourcePath(ByVal New_SourcePath As String)
     
       MstrPictureFromURL = New_SourcePath
        ' If program is in run mode, and the URL string
        ' is not empty, begin the download.
       AsyncRead New_SourcePath, vbAsyncTypePicture, _
            "SourcePath"

 PropertyChanged "SourcePath"
End Property