An exception of type System.FormatException occurred in mscorlib.dll but was not handled in user c

Hello,

            I dont understand why I am getting this issue even there is no type/data conversion error in my code. I just converted from 2.0 to 4.0 and with latest third party controls. I am getting the exception in the master page (design) exactly in the <td> which is given below. Please help

 MainMaster.master

 </tr>
                                    <tr> 
                                        <td class="banner" style="background-image: url(<%= LeftBorder() %>);">&nbsp;</td> 

Code Behind

Protected ReadOnly Property LeftBorder() As String
        Get
            Return ResolveUrl(LeftBorderSrc)
        End Get
    End Property

Private Const LeftBorderSrc As String = _
            "~/Images/MasterPages/LeftBorder.gif"