Are handles always zero or greater?

[aleph1]

I'm working on a system passing rigid body position data back and forth from a web worker, and ideally want to pass a number to tell the main thread to terminate the sequence. In order to do this reliably I was hoping to pass a negative number, but the relevant documentation for the JS API doesn't provide any additional detail about the RigidBody handle property beyond it being a Number. Does anyone know if the handle properly is ever assigned a negative number?