Custom signal not working

I'm dabbling with the SDK for sound synthesis, and came across the pd->sound->synth->setGenerator() tutorial which has an example of how to create a custom signal to modulate a synth's pitch based on the crank position.

However, when I try building the example code from the tutorial using SDK 2.1.1 (on Linux), the custom signal has no effect. I've tried with older versions of the SDK, and found that it does work up to version 1.12.3, and breaks from 1.13.0 onwards.

I've checked the release notes for the SDK and don't see anything about custom signals except their introduction in 1.12.0, so I don't think I've missed a change to the usage which would fix this (but happy to accept a correction on that).