"Engine Error: Unable to Shade polygon normals" Need help, Not a pirated version....

I am receiving the “Engine Error: Unable to Shade polygon normals (#################)”. Which i am aware means a pirated game. But I spent the $10 for the game because i always believe in supporting all game makers. I would like to know what i can do to get it to let me play Garry’s Mod.
I play in Steam offline mode most of the time. so it only has 2.4 hours recorded but i have atleast 60 hours spent into this game and i would like to make the amount of time longer that 60 hours.

Proof I own the game:http://steamcommunity.com/id/Cman1294/games/?tab=all&sort=name
First game on my list.

Does anyone know why this happened to a person with a legit copy?

Can someone tell me if this might be an issue of not updating the game for a little while?

Steam should auto update.
are you stopping steam from updating it

Yes in a way. I usually play offline mode cuz my internet hates steam very much. So yes i was stopping its auto update

It won’t work if you don’t update it.

I found that out. I just updated it… and its now working. Thank you for your quick responces.