Object reference not set to an instance of an object.

<h1>Object reference not set to an instance of an object.</h1><pre>System.Web.HttpUnhandledException (0x80004005): Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at AjaxControlToolkit.ToolkitScriptManager.GetScriptCombineAttributes(Assembly assembly)
   at AjaxControlToolkit.ToolkitScriptManager.IsScriptCombinable(ScriptEntry scriptEntry)
   at AjaxControlToolkit.ToolkitScriptManager.OnResolveScriptReference(ScriptReferenceEventArgs e)
   at System.Web.UI.ScriptManager.RegisterScripts()
   at System.Web.UI.ScriptManager.OnPagePreRenderComplete(Object sender, EventArgs e)
   at System.Web.UI.Page.OnPreRenderComplete(EventArgs e)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.HandleError(Exception e)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at ASP.actor_aspx.ProcessRequest(HttpContext context) in c:\windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root\187df388\cfb01e67\App_Web_o3g2hai3.9.cs:line 0
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)</pre>