后期绑定操作无法转换为具有LINQ的表达式树

我得到的错误是 后期绑定操作无法转换为表达式树 .

          <%
            For Each oCall In ViewData.Model
                %>
                <tr>
                    <td width="290"><a href="/Calls/Details/<%=oCall.CallID %>"><%=oCall.CallSubject %></a></td>
                    <td width="140"><%=oCall.CallReference %></td>
                    <td width="130"><%=oCall.CallType.CallTypeName %></td>
                    <td width="150"><%=oCall.DateOpened %></td>
                    <%
                        Dim assigned As String
                        Dim db As New CustomerServicesModelDataContext

                        Dim tStore = (From a In db.vStores _
                                      Where a.CompanyID = oCall.CompanyID _
                                      And a.StoreNumber = oCall.StoreID _
                                      Select a).SingleOrDefault()
                        assigned = tStore.Store
                        %>
                    <td width="175">
                        <%= assigned%>
                    </td>
                </tr>
                <%
            Next

            %>

错误发生两次。首先是oCall.CompanyID,然后是oCall.StoreID。

我错过什么了吗?

谢谢你的帮助。