Request.ServerVariables["HTTP_POST"] throws an exception

Hi Guys,
I've come across an exception thats being recorded by my Log4Net database about once a second in my ASP.NET webapplication. The error generated is:

System.NullReferenceException: Object reference not set to an instance of an object.     at ASP.global_asax.Application_BeginRequest(Object sender, EventArgs e) in d:\inetpub\wwwroot\mywebsitename\Global.asax:line 37     at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()     at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

In my Global.asax file line 37 reads:
string[] hostParts = Request.ServerVariables["HTTP_HOST"].Split('.');

I use this bit of code to get the country domain extension of the url, and then set the UIculture to the correct language.

Isn't the HTTP_POST variable always set in the header of a web request?