SqlSessionState:无法强制转换System类型的对象.DBNull到System.Byte[]类型

偶尔会出现以下错误。我还没能复制它。我试过登出并导航到一个需要身份验证的页面，但它没有抛出这个异常。

到目前为止，这个异常只发生在页面加载后立即调用的web服务上。(所以页面加载发生，然后onready事件开始加载一些数据与ajax调用)

我们正在使用带有SQL server会话存储的web表单。

Unable to cast object of type 'System.DBNull' to type 'System.Byte[]'.
at System.Web.SessionState.SqlSessionStateStore.DoGet(HttpContext context, String id, Boolean getExclusive, Boolean& locked, TimeSpan& lockAge, Object& lockId, SessionStateActions& actionFlags) at System.Web.SessionState.SqlSessionStateStore.GetItemExclusive(HttpContext context, String id, Boolean& locked, TimeSpan& lockAge, Object& lockId, SessionStateActions& actionFlags) at System.Web.SessionState.SessionStateModule.GetSessionStateItem() at System.Web.SessionState.SessionStateModule.BeginAcquireState(Object source, EventArgs e, AsyncCallback cb, Object extraData) at System.Web.HttpApplication.AsyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

我能做些什么来解决这个问题?