OnEntitySpawned Error

I've got a simple plugin which I wrote to handle Heli entering/leaving and being killed by players. The below is causing an error, and I can't for the life of my figure out why!

Code:

 void OnEntitySpawned(BaseEntity entity)
        {
            if (!entity is BaseHelicopter) return;
            if (entity is BaseHelicopter)
            {
                rust.BroadcastChat("[server-name:" + "<color=#55aaff>" + "The Patrol Helicopter" + "</color>" + " is incoming...");
            }
        }

This is the error:

Code:

Failed to call hook 'OnEntitySpawned' on plugin 'HeliUtils v0.0.1' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Core.Libraries.Covalence.Formatter+Lexer.Lex (System.String text) [0x00000] in <filename unknown>:0
  at Oxide.Core.Libraries.Covalence.Formatter.Parse (System.String text) [0x00000] in <filename unknown>:0
  at Oxide.Core.Libraries.Covalence.Formatter.ToTreeFormat (System.String text, System.Collections.Generic.Dictionary`2 translations) [0x00000] in <filename unknown>:0
  at Oxide.Core.Libraries.Covalence.Formatter.ToUnity (System.String text) [0x00000] in <filename unknown>:0
  at Oxide.Game.Rust.Libraries.Server.Broadcast (System.String message, System.String prefix, UInt64 userId) [0x00000] in <filename unknown>:0
  at Oxide.Game.Rust.Libraries.Rust.BroadcastChat (System.String name, System.String message, System.String userId) [0x00000] in <filename unknown>:0
  at Oxide.Plugins.HeliUtils.OnEntitySpawned (.BaseEntity entity) [0x00000] in <filename unknown>:0
  at Oxide.Plugins.HeliUtils.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00000] in <filename unknown>:0
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00000] in <filename unknown>:0
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x00000] in <filename unknown>:0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00000] in <filename unknown>:0 

Any ideas?
[DOUBLEPOST=1509722624][/DOUBLEPOST]Nevermind - Think I've sorted it. Was missing an argument on BroadcastChat for chat prefix, I've just put null.