C# - How To Add A System.Xml.XmlDocument Type To Applications State

I am using Asp.net 3.5 and C#

I have to add an XmlDocument to my application state so that everytime my application doesnt access the XML file on my filesystem, I will add this at the Application_Start() function in Global.asax.cs

I am adding this to system state as :

protected void Application_Start(Object sender, EventArgs e)
{
string filePath = Server.MapPath("<path to my XML FILE>");

[code]...

In this code i try to load the xml file and if the file is not loaded due to any problem then i am wanting a null XmlDocument.I access this XmlDocument as :

XmlDocument xmlDoc = new XmlDocument();
xmlDoc = HttpContext.Current.Application["xmlDoc"];

the error i get while build is Cannot implicitly convert type 'object' to 'System.Xml.XmlDocument'. An explicit conversion exists

So How to assign the HttpContext.Current.Application["xmlDoc"] variable as System.Xml.XmlDocument ?