How to generate a Youtube ID in Go?

I'm assuming all I need to do is encode 2^64 as base64 to get a 11 character Youtube identifier. I created a Go program https://play.golang.org/p/2nuA3JxVMd0 but it has two issues:

1. The identifier is 12 characters instead of the expected 11
2. The encoded base64 suffix is "=" which means that it didn't have enough to encode?

So where am I going wrong?