How to configure Istanbul to ignore methods in Node.js classes

Question

I'm running nyc 15.1.0 with jasmine 4.6.0 to generate code coverage reports for specs for Node.js classes. How do I configure nyc to ignore all methods named "toString" in all classes? I tried adding "ignore-instance-method" and "ignore-class-method" to .nycrc but neither changes the generated report. (Adding a source comment with "istanbul ignore next" works, but I don't want to add it to every class). I know nyc is reading .nycrc because it picks up other configuration changes.

My .nycrc looks like this:

{
  "all": true,
  "ignore-class-method": ["toString"],
  ...
}

Thanks.