index.d.ts is a generated file and any changes will be overwritten on rebuild. Actual source location is: https://github.com/dcodeIO/protobuf.js/blob/master/src/rpc.js#L28

@param {Error|null} then, probably.

Oh, I see. It actually needs to be @param {Error} [error].

http://usejsdoc.org/tags-param.html#optional-parameters-and-default-values

But is it actually optional? It cannot be undefined. There is either an Error, or none (null).

thanks for the fix !