Can't detect whether Session variable exists

I'm trying to determine if a Session variable exists, but I'm getting the error:

System.NullReferenceException: Object reference not set to an instance of an object.

Code:

    // Check if the "company_path" exists in the Session context
    if (System.Web.HttpContext.Current.Session["company_path"].ToString() != null)
    {
        // Session exists, set it
        company_path = System.Web.HttpContext.Current.Session["company_path"].ToString();
    }
    else
    {
        // Session doesn't exist, set it to the default
        company_path = "/reflex/SMD";
    }

That is because the Session name "company_path" doesn't exist, but I can't detect it!