Very Strange Inner Exception issue in Application_Error Method

Hello,
I am using Log4net to log exceptions.
In my Global.asax in method Application_Error I have this line of code to log exceptions.
log.Error("Unhandled Application Error:" + ex.InnerException.ToString());
But it is giving me the following exception itself.

NullReferenceException was unhandled by user code.
Object reference not set to an instance of an object.
If I replace ex.InnerException with ex.Message then everything works fine. Why ex.Innerexception is giving me this problem?


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