Would I be able to use a "free drink" slip?

So, yesterday I went into the closest Starbucks and they were out of caramel and whipped cream (my order is usually a caramel ribbon crunch with extra whipped cream, so, ya know). It isn't unusual at all for them to be out of stuff especially with me ordering late in the day.

I guess one of the employees took pity on me and she wrote me a "1 free drink" slip and signed her name. It's not official by any means, its ripped from a piece of paper. I'm worried I'll bring it up to the Starbucks and that they'll be unable to to give me the free drink, therefore I'll look like an idiot. Is the slip useable?

Update: I used it and they accepted it. I am a very happy gal :]. The worker was so polite.