HttpContext.Current.Session is throwing exception(Object reference not set to an inst

[SujaShyam]

In our application we have to access session objects in business class. We are using HttpContext.Cur rent.Session to get the session value. In some cases it returns the value but mostly its throwing a null reference exception(Objec t reference not set to an instance of an object). We have the following code
Try

If HttpContext.Cur rent.Session("U sername") IsNot Nothing then
' Statements to be executed
End If
Catch ex As Exception
'Log to db
End Try

Here HttpContext.Cur rent.Session("U sername") is mostly throwing an exception "Object reference not set to an instance of an object"
While debugging we found that HttpContext.Cur rent itself is nothing.

Thanks. Any help on this will be appreciated.