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.Current.Session to get the session value. In some cases it returns the value but mostly its throwing a null reference exception(Object reference not set to an instance of an object). We have the following code
Try

If HttpContext.Current.Session("Username") IsNot Nothing then
' Statements to be executed
End If
Catch ex As Exception
'Log to db
End Try

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

Thanks. Any help on this will be appreciated.