Unlike older releases, the license keys are node locked now. You must use a serial number that has a published set of corresponding licenses. Right now, there are 5 sets. 2 for 7 mode, and 3 for cluster mode. In the documentation some are listed as being for esx vs workstation, the serials numbers and license keys will work on either platform. Also, if you need more 7 mode instances, the C-mode key sets also work in 7 mode.
The only way to use some other random serial number, is to build it on 8.1.x, apply all the 8.1.x keys, and then perform an OnTap upgrade within the simulator.