`pbjs` cannot parse negative numbers

[endel]

Hey. Is this an expected behavior?

$ pbjs myfile.proto
/usr/local/lib/node_modules/protobufjs/cli/pbjs.js:74
            throw err;
            ^

Error: illegal id '-1' (line 76)

myfile.proto

enum Something {
	VALUE = -1;
}

I'm using the last v6.0.0. Cheers!